host/taint: pin what "resolved" must mean before F11-1 is implemented

Round 12 re-examined the deferral and agreed it holds while evaluate()
is audit-only, and that the recorded rule closes the path without
unbounding Pulse-emulated apps — but only under one reading of
"a node whose Client cannot be resolved at all".

The trap is worth writing down before anyone implements it: reading
"resolved" as "a unique Client object exists" passes for a unique Client
with sec_pid = None, which supplies no protected identity and leaves
exactly the self-claimed-PID hole the rule exists to close. It has to
mean an unambiguous Client yielding Some(pipewire.sec.pid), taken before
pipewire-pulse suppression.

That also means the §5.1 matrix needs five Client cases rather than two:
absent, ambiguous, unique-but-pid-less, resolved-native, and
resolved-to-pipewire-pulse. The pid-less row is the one that
distinguishes the two readings and the one a two-case matrix skips
without saying so.

Docs only. Still deferred, still to be decided with matrix data in hand.
220 tests green, clippy clean.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Cargo.toml

@@ -32,7 +32,7 @@ name = "pixelpass"
 path = "src/main.rs"
 
 [dependencies]
-iroh = "1.0.0-rc.0"
+iroh = "1.0.2"
 tokio = { version = "1", features = ["macros", "rt-multi-thread", "io-util", "net", "signal", "process", "sync", "time"] }
 tokio-util = { version = "0.7", features = ["io"] }
 clap = { version = "4", features = ["derive"] }
@@ -48,7 +48,7 @@ ashpd = { version = "0.9", default-features = false, features = ["tokio"] }
 pipewire = "0.9"
 x11rb = { version = "0.13", default-features = false, features = ["allow-unsafe-code"] }
 uuid = { version = "1", features = ["v4"] }
-iroh-tickets = "1.0.0-rc.0"
+iroh-tickets = "1.0.0"
 dialoguer = { version = "0.12", default-features = false }
 arboard = { version = "3", default-features = false, features = ["wayland-data-control"] }
 ureq = { version = "3", default-features = false, features = ["rustls"] }

README.md

@@ -23,6 +23,8 @@ Working:
 - Audio capture of the default sink's monitor, with optional per-app
   routing (`--app <name>`)
 - `--repair` cleanup of orphaned PipeWire state left by a crashed host
+- `--doctor` environment diagnostic (capture/encode deps, VA-API H.264,
+  viewer player, relay reachability) — see [Diagnostics](#diagnostics)
 - iroh QUIC bi-stream tunnel, direct-UDP and relay paths both verified
 - Interactive Host/View menu with clipboard auto-copy and mpv/VLC picker
 - Headless mode for scripts (`pixelpass <ticket>`)
@@ -135,6 +137,35 @@ sudo pacman -S vlc vlc-plugin-dvb vlc-plugin-ffmpeg
 If the viewer is running on battery, set the CPU governor to performance
 or balanced — power-saver can choke even hardware-decoded 1080p H.264.
 
+## Diagnostics
+
+`pixelpass --doctor` prints a one-shot report of everything the above
+requirements cover and exits — run it on any machine before a real session:
+
+```sh
+pixelpass --doctor
+```
+
+It checks, and prints a `✓ / ! / ✗` line for each:
+
+- **display server** — Wayland vs. X11 (autodetected), the raw session env
+  vars, and the X server's vendor/version (so an xlibre server is visible)
+- **capture** — the GStreamer tools plus the source element for your backend
+  (`pipewiresrc` on Wayland, `ximagesrc` on X11)
+- **encode** — whether hardware H.264 works (the `vah264enc` plugin, a DRM
+  render node, and a VA-API H.264 *encode* entrypoint via `vainfo`), and
+  whether the software `x264enc` fallback is available. This is the usual
+  culprit when a viewer "can't connect": a GPU with no H.264 encode entrypoint
+  produces no video under the default encoder — the report tells you to host
+  with `--no-hwencode`
+- **mux / audio** — the TS mux + AAC + PulseAudio tail, and `pactl`
+- **viewer** — whether `mpv` or `vlc` is installed
+- **network** — binds a real endpoint and checks a relay is reachable
+
+Each failing line includes a distro-aware install hint, and the closing summary
+says whether the machine can host and how. The exit code is non-zero if any
+hard requirement is missing, so it can gate a script or CI.
+
 ## Build
 
 ```sh

src/cli.rs

@@ -105,11 +105,31 @@ pub struct Cli {
     #[arg(long)]
     pub repair: bool,
 
+    /// Print an environment diagnostic report (display server, capture/encode
+    /// dependencies, VA-API H.264 support, viewer player, relay reachability),
+    /// then exit. Use this to check a machine can host or view before a real
+    /// session — especially to confirm hardware H.264 encode works, since a GPU
+    /// without it silently produces no video under the default encoder.
+    #[arg(long)]
+    pub doctor: bool,
+
     /// Re-run the bandwidth pre-flight test, save the result, then exit.
     /// Use this if your connection has changed (new ISP, moved house, etc.)
     /// or if the previously saved test result is stale.
     #[arg(long)]
     pub reconfigure: bool,
+
+    /// Run the read-only audio-exclusion dry-run audit against the live
+    /// PipeWire graph, then exit on ctrl-c. Emits one JSON object per line to
+    /// stderr (or to `PIXELPASS_AUDIO_AUDIT_FILE`) describing which audio
+    /// streams would be eligible for a screen share and why the rest would not.
+    /// Creates no links and changes no routing.
+    ///
+    /// Hidden: this is development instrumentation for the screen-share audio
+    /// exclusion work (impl plan phase 5), not a user-facing feature, and the
+    /// record schema is free to change until phase 6 fixes it.
+    #[arg(long, hide = true)]
+    pub audit_audio: bool,
 }
 
 #[derive(ValueEnum, Clone, Copy, Debug)]

src/common/deps.rs

@@ -56,12 +56,7 @@ fn require(bin: &str) -> Result<PathBuf> {
 }
 
 fn require_gst_element(name: &str) -> Result<()> {
-    let ok = Command::new("gst-inspect-1.0")
-        .args(["--exists", name])
-        .status()
-        .map(|s| s.success())
-        .unwrap_or(false);
-    if !ok {
+    if !gst_element_exists(name) {
         bail!(
             "GStreamer element `{name}` not available.\n{}",
             install_hint_for_gst_element(name)
@@ -70,7 +65,17 @@ fn require_gst_element(name: &str) -> Result<()> {
     Ok(())
 }
 
-fn which(bin: &str) -> Option<PathBuf> {
+/// Whether a GStreamer element is registered, via `gst-inspect-1.0 --exists`.
+/// Non-bailing counterpart to [`require_gst_element`] for the `doctor` report.
+pub(crate) fn gst_element_exists(name: &str) -> bool {
+    Command::new("gst-inspect-1.0")
+        .args(["--exists", name])
+        .status()
+        .map(|s| s.success())
+        .unwrap_or(false)
+}
+
+pub(crate) fn which(bin: &str) -> Option<PathBuf> {
     let path = std::env::var_os("PATH")?;
     for dir in std::env::split_paths(&path) {
         let candidate = dir.join(bin);
@@ -81,7 +86,7 @@ fn which(bin: &str) -> Option<PathBuf> {
     None
 }
 
-fn install_hint_for_bin(bin: &str) -> String {
+pub(crate) fn install_hint_for_bin(bin: &str) -> String {
     let distro = detect_distro();
     let pkg = match bin {
         "gst-launch-1.0" | "gst-inspect-1.0" => match distro.as_deref() {
@@ -113,7 +118,7 @@ fn install_hint_for_bin(bin: &str) -> String {
     install_command(&distro, pkg)
 }
 
-fn install_hint_for_gst_element(name: &str) -> String {
+pub(crate) fn install_hint_for_gst_element(name: &str) -> String {
     let distro = detect_distro();
     let pkg = match name {
         "pipewiresrc" => match distro.as_deref() {
@@ -210,7 +215,7 @@ fn install_command(distro: &Option<String>, pkg: &str) -> String {
     format!("Install hint: {cmd}")
 }
 
-fn detect_distro() -> Option<String> {
+pub(crate) fn detect_distro() -> Option<String> {
     let contents = std::fs::read_to_string("/etc/os-release").ok()?;
     for line in contents.lines() {
         if let Some(rest) = line.strip_prefix("ID=") {

src/common/signal.rs

@@ -1,5 +1,15 @@
+use anyhow::{Context, Result};
+use tokio::signal::unix::{Signal, SignalKind};
 use tokio_util::sync::CancellationToken;
 
+/// A stream of SIGTERMs, for the callers that need to shut down cleanly when
+/// something other than a human at a terminal asks them to (`timeout`, a test
+/// harness, a service manager). Ctrl-c alone covers only the interactive case.
+pub fn terminate_stream() -> Result<Signal> {
+    tokio::signal::unix::signal(SignalKind::terminate())
+        .context("could not install a SIGTERM handler")
+}
+
 /// Install a ctrl-c handler that triggers the returned token.
 ///
 /// The first ctrl-c cancels gracefully; a second ctrl-c terminates the process.

src/doctor.rs

@@ -0,0 +1,648 @@
+//! `pixelpass doctor` — environment diagnostics.
+//!
+//! Screen-share failures are usually not pixelpass bugs but environment gaps:
+//! a missing GStreamer plugin, an X vs. Wayland mismatch, or — the common one —
+//! a GPU/driver with no working VA-API H.264 encoder, so the default
+//! `vah264enc` pipeline never produces a byte and the viewer "can't connect."
+//! `doctor` probes all of that up front and prints one actionable report, so a
+//! remote tester can read it over a call instead of us guessing from logs. It
+//! also validates any X11/Wayland test environment we stand up.
+//!
+//! Unlike [`crate::common::deps::check_host_binaries`], which bails on the first
+//! missing dependency, doctor runs *every* check and reports them together — a
+//! diagnostic wants the whole picture, not the first failure.
+
+use anyhow::Result;
+use std::time::Duration;
+
+use crate::common::deps;
+use crate::common::display::DisplayServer;
+use crate::common::endpoint;
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum Status {
+    /// Working as needed.
+    Ok,
+    /// Degraded but not fatal (e.g. a fallback path is available).
+    Warn,
+    /// Screen-sharing will not work until this is fixed.
+    Fail,
+    /// Neutral fact, no judgement.
+    Info,
+}
+
+impl Status {
+    fn icon(self) -> char {
+        match self {
+            Self::Ok => '✓',
+            Self::Warn => '!',
+            Self::Fail => '✗',
+            Self::Info => '·',
+        }
+    }
+}
+
+/// One line in the report: a status, a short label, a detail, and an optional
+/// remediation hint printed on its own indented line.
+pub struct Check {
+    pub status: Status,
+    pub label: String,
+    pub detail: String,
+    pub hint: Option<String>,
+}
+
+impl Check {
+    fn new(status: Status, label: impl Into<String>, detail: impl Into<String>) -> Self {
+        Self {
+            status,
+            label: label.into(),
+            detail: detail.into(),
+            hint: None,
+        }
+    }
+
+    fn ok(label: impl Into<String>, detail: impl Into<String>) -> Self {
+        Self::new(Status::Ok, label, detail)
+    }
+    fn warn(label: impl Into<String>, detail: impl Into<String>) -> Self {
+        Self::new(Status::Warn, label, detail)
+    }
+    fn fail(label: impl Into<String>, detail: impl Into<String>) -> Self {
+        Self::new(Status::Fail, label, detail)
+    }
+    fn info(label: impl Into<String>, detail: impl Into<String>) -> Self {
+        Self::new(Status::Info, label, detail)
+    }
+
+    fn with_hint(mut self, hint: impl Into<String>) -> Self {
+        self.hint = Some(hint.into());
+        self
+    }
+}
+
+/// Tally of the non-trivial statuses across every section.
+#[derive(Debug, Default, Clone, Copy, PartialEq, Eq)]
+pub struct Summary {
+    pub fails: usize,
+    pub warns: usize,
+}
+
+/// A named group of checks, printed under a header.
+struct Section {
+    name: &'static str,
+    checks: Vec<Check>,
+}
+
+/// Run all diagnostics and print the report. Always prints; the process exit
+/// code is non-zero only when a hard failure (a `Fail`) was found, so scripts
+/// and CI can gate on it while a human still sees everything.
+pub async fn run(relay: Option<String>) -> Result<()> {
+    let display = DisplayServer::detect();
+
+    let sections = vec![
+        system_section(display),
+        capture_section(display),
+        encode_section(),
+        mux_audio_section(),
+        viewer_section(),
+        network_section(relay.as_deref()).await,
+    ];
+
+    print_report(&sections);
+
+    let summary = summarize(sections.iter().flat_map(|s| s.checks.iter()));
+    print_summary(summary, &sections);
+
+    if summary.fails > 0 {
+        std::process::exit(1);
+    }
+    Ok(())
+}
+
+// ── sections ──────────────────────────────────────────────────────────────
+
+fn system_section(display: DisplayServer) -> Section {
+    let mut checks = vec![
+        Check::info(
+            "pixelpass",
+            format!("{} (gui: {})", env!("CARGO_PKG_VERSION"), gui_built()),
+        ),
+        Check::info("distro", distro_detail()),
+        display_check(display),
+    ];
+
+    // Probe the actual X server when one is reachable — this is where an xlibre
+    // vs. Xorg difference (the thing we most want to see on a tester's box)
+    // shows up. Skip it on a pure Wayland session with no X at all.
+    if display == DisplayServer::X11 || std::env::var_os("DISPLAY").is_some() {
+        checks.push(x_server_check());
+    }
+
+    Section {
+        name: "System",
+        checks,
+    }
+}
+
+fn capture_section(display: DisplayServer) -> Section {
+    let mut checks = vec![
+        bin_check("gst-launch-1.0", "gstreamer tools"),
+        bin_check("gst-inspect-1.0", "gstreamer tools"),
+    ];
+
+    match display {
+        DisplayServer::Wayland => {
+            checks.push(gst_check("pipewiresrc", "Wayland capture"));
+        }
+        DisplayServer::X11 => {
+            checks.push(gst_check("ximagesrc", "X11 capture"));
+            checks.push(match deps::which("xwininfo") {
+                Some(p) => Check::ok("window picker", p.display().to_string())
+                    .with_hint("needed only for `--window` (share a single window)"),
+                None => Check::info("window picker", "xwininfo not found")
+                    .with_hint("optional — only `--window` needs it"),
+            });
+        }
+        DisplayServer::Unknown => {
+            checks.push(
+                Check::info("capture backend", "unknown — cannot probe a source element")
+                    .with_hint("force one with `--display-server x11|wayland` when hosting"),
+            );
+        }
+    }
+
+    Section {
+        name: "Capture (host)",
+        checks,
+    }
+}
+
+fn encode_section() -> Section {
+    Section {
+        name: "Encode",
+        checks: vec![hardware_encode_check(), software_encode_check()],
+    }
+}
+
+/// The load-bearing check for the common "viewer can't connect" report: the
+/// default host pipeline uses `vah264enc`, which needs both the GStreamer VA
+/// plugin *and* a GPU/driver that actually exposes an H.264 encode entrypoint.
+/// A box with the plugin but no encode entrypoint (or no render node) produces
+/// no video — the exact silent failure `--no-hwencode` works around.
+fn hardware_encode_check() -> Check {
+    if !deps::gst_element_exists("vah264enc") {
+        return Check::warn("hardware H.264", "vah264enc plugin not installed").with_hint(format!(
+            "{} — or just host with `--no-hwencode` (software x264)",
+            deps::install_hint_for_gst_element("vah264enc")
+        ));
+    }
+
+    if !has_render_node() {
+        return Check::warn(
+            "hardware H.264",
+            "vah264enc present, but no DRM render node (/dev/dri/renderD*)",
+        )
+        .with_hint("GPU encode is unavailable here — host with `--no-hwencode`");
+    }
+
+    match vainfo_output() {
+        Some(out) if vainfo_has_h264_encode(&out) => Check::ok(
+            "hardware H.264",
+            "VA-API H.264 encode available (vah264enc)",
+        ),
+        Some(_) => Check::warn(
+            "hardware H.264",
+            "vah264enc present, but VA-API reports no H.264 encode entrypoint",
+        )
+        .with_hint("this GPU/driver can't hardware-encode H.264 — host with `--no-hwencode`"),
+        None => Check::info(
+            "hardware H.264",
+            "vah264enc + render node present; couldn't confirm the VA-API encode entrypoint",
+        )
+        .with_hint("install `vainfo` (libva-utils) to verify, or just test a real host session"),
+    }
+}
+
+fn software_encode_check() -> Check {
+    if deps::gst_element_exists("x264enc") {
+        Check::ok("software H.264", "x264enc available (`--no-hwencode`)")
+    } else {
+        Check::warn("software H.264", "x264enc not installed").with_hint(format!(
+            "{} — the fallback for GPUs without VA-API H.264 encode",
+            deps::install_hint_for_gst_element("x264enc")
+        ))
+    }
+}
+
+fn mux_audio_section() -> Section {
+    // These live in plugins-bad/-good/-libav and plugins-base; all are required
+    // for either backend, so a miss here is a hard Fail.
+    let tail = [
+        "h264parse",
+        "mpegtsmux",
+        "aacparse",
+        "avenc_aac",
+        "pulsesrc",
+        "videoscale",
+    ];
+    let missing: Vec<&str> = tail
+        .iter()
+        .copied()
+        .filter(|e| !deps::gst_element_exists(e))
+        .collect();
+
+    let tail_check = if missing.is_empty() {
+        Check::ok("mux + audio tail", tail.join(", "))
+    } else {
+        Check::fail(
+            "mux + audio tail",
+            format!("missing: {}", missing.join(", ")),
+        )
+        .with_hint(deps::install_hint_for_gst_element(missing[0]))
+    };
+
+    Section {
+        name: "Mux / audio",
+        checks: vec![tail_check, bin_check("pactl", "pactl")],
+    }
+}
+
+fn viewer_section() -> Section {
+    let mpv = deps::which("mpv");
+    let vlc = deps::which("vlc");
+
+    let check = match (mpv, vlc) {
+        (Some(p), _) => Check::ok("player", format!("mpv ({})", p.display())),
+        (None, Some(p)) => Check::ok("player", format!("vlc ({})", p.display()))
+            .with_hint("mpv is the recommended player; vlc needs the dvb + ffmpeg plugins"),
+        (None, None) => Check::warn("player", "neither mpv nor vlc found")
+            .with_hint("a viewer needs one of them; the GUI launches mpv by default"),
+    };
+
+    Section {
+        name: "Viewer",
+        checks: vec![check],
+    }
+}
+
+/// Bind a real video-plane endpoint and wait briefly for a relay, mirroring
+/// what a host does. Directly relevant to "couldn't connect": if this machine
+/// can't reach a relay, hole-punching to a peer is unlikely to work either.
+async fn network_section(relay: Option<&str>) -> Section {
+    let check = match endpoint::bind(relay).await {
+        Ok(ep) => {
+            let online = tokio::time::timeout(Duration::from_secs(8), ep.online())
+                .await
+                .is_ok();
+            let relay_count = ep.addr().addrs.iter().filter(|a| a.is_relay()).count();
+            let where_ = relay.map(|r| format!(" ({r})")).unwrap_or_default();
+            // Close gracefully so iroh doesn't log a scary "Endpoint dropped
+            // without calling close" error into the middle of the report.
+            ep.close().await;
+
+            if online && relay_count > 0 {
+                Check::ok("relay", format!("home relay reachable{where_}"))
+            } else if online {
+                Check::warn(
+                    "relay",
+                    format!("endpoint online but no relay address{where_}"),
+                )
+                .with_hint(
+                    "n0 DNS discovery may still connect peers, but relay fallback is degraded",
+                )
+            } else {
+                Check::warn("relay", format!("no relay connected within 8s{where_}")).with_hint(
+                    "check connectivity/firewall; peers behind NAT rely on the relay to rendezvous",
+                )
+            }
+        }
+        Err(e) => Check::fail("relay", format!("could not bind endpoint: {e}")),
+    };
+
+    Section {
+        name: "Network",
+        checks: vec![check],
+    }
+}
+
+// ── small check builders ────────────────────────────────────────────────────
+
+fn bin_check(bin: &str, label: &str) -> Check {
+    match deps::which(bin) {
+        Some(p) => Check::ok(label, format!("{bin} ({})", p.display())),
+        None => Check::fail(label, format!("{bin} not found on PATH"))
+            .with_hint(deps::install_hint_for_bin(bin)),
+    }
+}
+
+fn gst_check(element: &str, label: &str) -> Check {
+    if deps::gst_element_exists(element) {
+        Check::ok(label, element.to_string())
+    } else {
+        Check::fail(
+            label,
+            format!("GStreamer element `{element}` not available"),
+        )
+        .with_hint(deps::install_hint_for_gst_element(element))
+    }
+}
+
+fn display_check(display: DisplayServer) -> Check {
+    let env = display_env_summary();
+    match display {
+        DisplayServer::Wayland => Check::ok("display server", format!("Wayland ({env})")),
+        DisplayServer::X11 => Check::ok("display server", format!("X11 ({env})")),
+        DisplayServer::Unknown => Check::fail("display server", format!("undetected ({env})"))
+            .with_hint(
+                "no WAYLAND_DISPLAY/DISPLAY/XDG_SESSION_TYPE — capture can't start; \
+                 run inside a graphical session or pass `--display-server`",
+            ),
+    }
+}
+
+/// Connect to the X server and report its vendor + version. This is how an
+/// xlibre server distinguishes itself from stock Xorg (vendor string / release
+/// number), which is exactly what we want to see on a tester's machine.
+fn x_server_check() -> Check {
+    use x11rb::connection::Connection;
+    match x11rb::connect(None) {
+        Ok((conn, _screen)) => {
+            let setup = conn.setup();
+            let vendor = String::from_utf8_lossy(&setup.vendor);
+            let detail = format!(
+                "vendor \"{}\", protocol {}.{}, release {}",
+                vendor.trim(),
+                setup.protocol_major_version,
+                setup.protocol_minor_version,
+                setup.release_number,
+            );
+            let label = "X server";
+            if vendor.to_lowercase().contains("xlibre") {
+                Check::info(label, format!("XLibre — {detail}"))
+            } else {
+                Check::info(label, detail)
+            }
+        }
+        Err(_) => Check::info("X server", "DISPLAY set but the X server is unreachable"),
+    }
+}
+
+// ── environment helpers ─────────────────────────────────────────────────────
+
+fn gui_built() -> &'static str {
+    if cfg!(feature = "gui") { "yes" } else { "no" }
+}
+
+fn distro_detail() -> String {
+    let id = deps::detect_distro();
+    let pretty = os_release_field("PRETTY_NAME");
+    match (id, pretty) {
+        (Some(id), Some(p)) => format!("{id} ({p})"),
+        (Some(id), None) => id,
+        (None, Some(p)) => p,
+        (None, None) => "unknown".to_string(),
+    }
+}
+
+fn os_release_field(key: &str) -> Option<String> {
+    let contents = std::fs::read_to_string("/etc/os-release").ok()?;
+    for line in contents.lines() {
+        if let Some(rest) = line.strip_prefix(&format!("{key}=")) {
+            return Some(rest.trim_matches('"').to_string());
+        }
+    }
+    None
+}
+
+fn display_env_summary() -> String {
+    let mut parts = Vec::new();
+    for var in [
+        "WAYLAND_DISPLAY",
+        "DISPLAY",
+        "XDG_SESSION_TYPE",
+        "XDG_CURRENT_DESKTOP",
+    ] {
+        if let Some(v) = std::env::var_os(var) {
+            parts.push(format!("{var}={}", v.to_string_lossy()));
+        }
+    }
+    if parts.is_empty() {
+        "no display env vars set".to_string()
+    } else {
+        parts.join(", ")
+    }
+}
+
+fn has_render_node() -> bool {
+    let Ok(entries) = std::fs::read_dir("/dev/dri") else {
+        return false;
+    };
+    entries
+        .flatten()
+        .any(|e| e.file_name().to_string_lossy().starts_with("renderD"))
+}
+
+fn vainfo_output() -> Option<String> {
+    deps::which("vainfo")?;
+    let out = std::process::Command::new("vainfo").output().ok()?;
+    // vainfo prints its profile/entrypoint table to stdout; some builds also
+    // spill driver banners to stderr. Concatenate both so parsing is robust.
+    let mut s = String::from_utf8_lossy(&out.stdout).into_owned();
+    s.push_str(&String::from_utf8_lossy(&out.stderr));
+    Some(s)
+}
+
+/// Pure: does a `vainfo` dump advertise an H.264 *encode* entrypoint? vainfo
+/// lists one `VAProfile… : VAEntrypoint…` pair per line; hardware H.264 encode
+/// is any `VAProfileH264*` profile paired with an `EncSlice`/`EncSliceLP`
+/// entrypoint. VLD-only H.264 (decode) does not count.
+fn vainfo_has_h264_encode(output: &str) -> bool {
+    output.lines().any(|line| {
+        line.contains("VAProfileH264")
+            && (line.contains("VAEntrypointEncSlice") || line.contains("VAEntrypointEncSliceLP"))
+    })
+}
+
+// ── reporting ───────────────────────────────────────────────────────────────
+
+fn print_report(sections: &[Section]) {
+    println!("pixelpass doctor\n");
+    for section in sections {
+        println!("{}", section.name);
+        for check in &section.checks {
+            println!(
+                "  {} {:<16}  {}",
+                check.status.icon(),
+                check.label,
+                check.detail
+            );
+            if let Some(hint) = &check.hint {
+                println!("        → {hint}");
+            }
+        }
+        println!();
+    }
+}
+
+fn summarize<'a>(checks: impl Iterator<Item = &'a Check>) -> Summary {
+    let mut summary = Summary::default();
+    for check in checks {
+        match check.status {
+            Status::Fail => summary.fails += 1,
+            Status::Warn => summary.warns += 1,
+            Status::Ok | Status::Info => {}
+        }
+    }
+    summary
+}
+
+fn print_summary(summary: Summary, sections: &[Section]) {
+    let hosting = hosting_verdict(sections);
+    let counts = match (summary.fails, summary.warns) {
+        (0, 0) => "all checks passed".to_string(),
+        (0, w) => format!("{w} warning{}", plural(w)),
+        (f, 0) => format!("{f} failure{}", plural(f)),
+        (f, w) => format!("{f} failure{}, {w} warning{}", plural(f), plural(w)),
+    };
+    println!("Summary: {counts}. {hosting}");
+}
+
+fn plural(n: usize) -> &'static str {
+    if n == 1 { "" } else { "s" }
+}
+
+/// A one-line verdict on whether this box can host, and how. Reads the actual
+/// encode + capture checks rather than the raw tally so the advice is specific.
+fn hosting_verdict(sections: &[Section]) -> String {
+    let find = |section: &str, label: &str| -> Option<Status> {
+        sections
+            .iter()
+            .find(|s| s.name == section)?
+            .checks
+            .iter()
+            .find(|c| c.label == label)
+            .map(|c| c.status)
+    };
+
+    let hw = find("Encode", "hardware H.264");
+    let sw_ok = find("Encode", "software H.264") == Some(Status::Ok);
+    let capture_broken = sections
+        .iter()
+        .find(|s| s.name == "Capture (host)")
+        .map(|s| s.checks.iter().any(|c| c.status == Status::Fail))
+        .unwrap_or(false);
+
+    if capture_broken {
+        "Hosting will fail: the capture backend is incomplete (see Capture above).".to_string()
+    } else if hw == Some(Status::Ok) {
+        "Hosting will work (hardware H.264 encode).".to_string()
+    } else if hw == Some(Status::Info) && sw_ok {
+        // Plugin + render node present but VA-API unverified (no vainfo): the
+        // default encoder is likely fine; `--no-hwencode` is the safe fallback.
+        "Hosting should work (hardware H.264 likely; `--no-hwencode` is the fallback).".to_string()
+    } else if sw_ok {
+        "Hosting should work with `--no-hwencode` (software H.264 encode).".to_string()
+    } else {
+        "Hosting may fail: no working H.264 encoder found (see Encode above).".to_string()
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn vainfo_detects_h264_encode_entrypoint() {
+        // Realistic AMD/RADV-style dump: H.264 has both decode (VLD) and encode.
+        let dump = "\
+      VAProfileH264Main               : VAEntrypointVLD
+      VAProfileH264Main               : VAEntrypointEncSlice
+      VAProfileH264High               : VAEntrypointVLD
+      VAProfileHEVCMain               : VAEntrypointEncSlice";
+        assert!(vainfo_has_h264_encode(dump));
+    }
+
+    #[test]
+    fn vainfo_low_power_encode_counts() {
+        let dump = "VAProfileH264ConstrainedBaseline: VAEntrypointEncSliceLP";
+        assert!(vainfo_has_h264_encode(dump));
+    }
+
+    #[test]
+    fn vainfo_decode_only_h264_is_not_encode() {
+        // Decode-only H.264 (VLD) plus HEVC encode must NOT be read as H.264
+        // encode — this is exactly the "default encoder fails" case.
+        let dump = "\
+      VAProfileH264Main               : VAEntrypointVLD
+      VAProfileH264High               : VAEntrypointVLD
+      VAProfileHEVCMain               : VAEntrypointEncSlice";
+        assert!(!vainfo_has_h264_encode(dump));
+    }
+
+    #[test]
+    fn vainfo_empty_is_not_encode() {
+        assert!(!vainfo_has_h264_encode(""));
+    }
+
+    #[test]
+    fn summarize_counts_fails_and_warns_only() {
+        let checks = [
+            Check::ok("a", "x"),
+            Check::info("b", "x"),
+            Check::warn("c", "x"),
+            Check::warn("d", "x"),
+            Check::fail("e", "x"),
+        ];
+        let summary = summarize(checks.iter());
+        assert_eq!(summary, Summary { fails: 1, warns: 2 });
+    }
+
+    #[test]
+    fn hosting_verdict_prefers_hardware_then_software() {
+        let hw = vec![Section {
+            name: "Encode",
+            checks: vec![
+                Check::ok("hardware H.264", "ok"),
+                Check::ok("software H.264", "ok"),
+            ],
+        }];
+        assert!(hosting_verdict(&hw).contains("hardware"));
+
+        let sw = vec![Section {
+            name: "Encode",
+            checks: vec![
+                Check::warn("hardware H.264", "no"),
+                Check::ok("software H.264", "ok"),
+            ],
+        }];
+        assert!(sw_verdict_uses_no_hwencode(&hosting_verdict(&sw)));
+
+        let none = vec![Section {
+            name: "Encode",
+            checks: vec![
+                Check::warn("hardware H.264", "no"),
+                Check::warn("software H.264", "no"),
+            ],
+        }];
+        assert!(hosting_verdict(&none).contains("may fail"));
+    }
+
+    fn sw_verdict_uses_no_hwencode(v: &str) -> bool {
+        v.contains("--no-hwencode")
+    }
+
+    #[test]
+    fn capture_failure_dominates_verdict() {
+        let sections = vec![
+            Section {
+                name: "Capture (host)",
+                checks: vec![Check::fail("X11 capture", "missing")],
+            },
+            Section {
+                name: "Encode",
+                checks: vec![Check::ok("hardware H.264", "ok")],
+            },
+        ];
+        assert!(hosting_verdict(&sections).contains("capture"));
+    }
+}

src/host/aec/mod.rs

@@ -0,0 +1,311 @@
+//! Phase 4 — the AEC identity validation state machine (impl plan §4, design
+//! v3.4 §5.2/§5.3).
+//!
+//! peerspeak's echo canceller (`module-echo-cancel`) creates four graph nodes
+//! that all carry `pulse.module.id == <the index pactl returned>`, and the
+//! playback leg among them is a `Stream/Output/Audio` node wired straight to
+//! the speakers — a fan-out candidate that would copy the whole remote call
+//! into the share unless it is excluded (v3.4 §5.2, measured ≈desktop level).
+//! The taint engine (phase 2) already excludes it *given* the module index in
+//! [`ExclusionCtx::aec_module_id`](crate::host::taint::ExclusionCtx); this
+//! module is what decides, at runtime and fail-closed, whether that index may
+//! be trusted and handed over.
+//!
+//! **Why a state machine and not a one-shot check (v3.4 §5.3).** The identity
+//! is an *observed correlation on PipeWire 1.6.8*, not a documented contract,
+//! and a start-time enumeration races in both directions: peerspeak's
+//! `enable()` returns before the playback hazard leg is even in the graph, and
+//! pixelpass's capture spawns lazily on the first viewer, at a moment peerspeak
+//! does not control. So validation is a bounded epoch, and the identity can be
+//! *lost* mid-share (the module unloads) as well as *gained*.
+//!
+//! **The two traps this is shaped around:**
+//!
+//! - **Revocation is loss of the whole module identity, not one leg corking**
+//!   (v3.4 §5.3). Each [`AecValidator::observe`] rescans the snapshot for *any*
+//!   node bearing the index; [`AecState::Validated`] drops to
+//!   [`AecState::Revoked`] only when that set becomes **empty**. A single leg
+//!   corking or relinking (still ≥1 present) stays `Validated` — getting this
+//!   wrong turns a normal cork into a spurious share-wide audio stop.
+//! - **Module indices are reused verbatim across unload/reload** (v3.4 §5.2
+//!   correction 3 — both a reload's module index *and* its `node.link-group`
+//!   came back byte-identical, and node ids were recycled *and reassigned
+//!   across legs*). So [`AecState::Failed`] and [`AecState::Revoked`] are
+//!   **sticky terminal**: a later node reappearing with the same index does
+//!   **not** un-revoke and alias onto the new module. A genuine reload gets a
+//!   *fresh* [`AecValidator`] (peerspeak re-tells pixelpass the index on every
+//!   load), never a resurrected one.
+//!
+//! **Scope.** This is the validation state machine + `--aec` parsing only.
+//! Foreign / second-AEC detection (a non-owned `echo-cancel-*` group, v3.4
+//! §5.4 / D3) and the `foreign_aec_warning`/`aec_failed`/`aec_revoked` status
+//! *events* are phase 6's, which reads this machine's [`AecState`]. Wiring the
+//! parsed [`AecConfig`] out of the CLI and calling [`AecValidator::observe`]
+//! in the recompute loop is integration (phases 5/8). The node-side
+//! `pulse.module.id` parse (JSON-number-vs-string, u64-not-u32) is phase 3's
+//! adapter; this module consumes the already-parsed
+//! [`NodeProps::pulse_module_id`](crate::host::taint::snapshot::NodeProps).
+
+#![allow(dead_code)] // Wired into `--aec` parsing + the recompute loop by later phases.
+
+#[cfg(test)]
+mod tests;
+
+use crate::host::observer::Millis;
+use crate::host::taint::snapshot::GraphSnapshot;
+
+/// The parsed `--aec=off|pulse-module:<idx>` argument (decision D5).
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+pub enum AecConfig {
+    /// `--aec=off` — peerspeak's AEC is not in play, so there is nothing to
+    /// exclude and fan-out proceeds with no AEC identity. Not the same as an
+    /// *absent* argument (that default is the caller's; see [`parse_aec_arg`]).
+    Off,
+    /// `--aec=pulse-module:<idx>` — validate this live module index before
+    /// trusting it. The index is compared as `u64`, never `u32` (v3.4 §5.2).
+    PulseModule(u64),
+}
+
+/// Why an `--aec` argument was rejected. Rejection is fatal at the CLI edge —
+/// there is no fail-closed *default* index, because a wrong index would exclude
+/// the wrong node (or nothing), so a malformed value must not silently become
+/// "no AEC".
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+pub enum AecParseError {
+    /// The value was empty.
+    Empty,
+    /// Not `off` and not `pulse-module:...`.
+    UnknownForm,
+    /// `pulse-module:` with nothing after the colon.
+    MissingIndex,
+    /// The index was not a bare `u64` decimal (sign, whitespace, non-digit, or
+    /// `> u64::MAX`).
+    InvalidIndex,
+}
+
+/// Parse one `--aec` value. `off` and `pulse-module:<idx>` are the only forms.
+///
+/// The index accepts values `> u32::MAX` (v3.4 §5.2: `pulse.module.id` sits
+/// next to the `object.serial` u32-truncation bug, so it is only ever compared
+/// as `u64`) and requires a **bare decimal** — stricter than Rust's [`u64`]
+/// parser, which also accepts a leading `+`. Rejected: any sign, surrounding or
+/// interior whitespace, non-decimal digits, and overflow. Matching is exact and
+/// case-sensitive: the argument is machine-generated by peerspeak from
+/// `EchoCancelGuard::module_index`, not typed by a user.
+///
+/// ⚠️ **Producer contract** (Codex phase-4 review, finding 5): because the
+/// grammar is narrower than Rust's parser, peerspeak must emit a bare decimal.
+/// `pactl load-module` returns an unsigned decimal, so the stored index is
+/// already canonical and no reachable value is rejected; if peerspeak ever
+/// changes how it formats the index it must canonicalize (`value.to_string()`),
+/// not widen this parser — the narrow grammar is the point.
+pub fn parse_aec_arg(value: &str) -> Result<AecConfig, AecParseError> {
+    if value.is_empty() {
+        return Err(AecParseError::Empty);
+    }
+    if value == "off" {
+        return Ok(AecConfig::Off);
+    }
+    if let Some(index) = value.strip_prefix("pulse-module:") {
+        if index.is_empty() {
+            return Err(AecParseError::MissingIndex);
+        }
+        // A bare decimal only: reject a leading sign (Rust's `u64` parser
+        // accepts `+7`), interior/surrounding whitespace, and any non-digit,
+        // before letting the parser catch overflow. Leading zeros are harmless.
+        if !index.bytes().all(|b| b.is_ascii_digit()) {
+            return Err(AecParseError::InvalidIndex);
+        }
+        return index
+            .parse::<u64>()
+            .map(AecConfig::PulseModule)
+            .map_err(|_| AecParseError::InvalidIndex);
+    }
+    Err(AecParseError::UnknownForm)
+}
+
+/// The validation epoch (v3.4 §5.3, verbatim).
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+pub enum AecState {
+    /// `--aec=off` — no AEC identity, fan-out proceeds with no exclusion.
+    /// Terminal.
+    NotConfigured,
+    /// Waiting for the first node bearing the index. **No fan-out occurs here**
+    /// — silence is the safe direction. Ends at `Validated` on first sight, or
+    /// `Failed` once the graph is fully enumerated and the bounded deadline
+    /// passes with the index never seen.
+    Validating,
+    /// The index was observed live. Fan-out is permitted, excluding that
+    /// identity transitively (phase 2 / v3.4 §6.1).
+    Validated,
+    /// The deadline expired with the index never observed. **Fail closed** — no
+    /// fan-out; the caller reports a capability failure rather than sharing.
+    /// Sticky terminal.
+    Failed,
+    /// The whole module identity disappeared mid-share (every node bearing the
+    /// index gone). **Stop fan-out now** and drop the owned link proxies; do
+    /// not keep the numeric index and hope, because it is reused. Sticky
+    /// terminal — see the module header's second trap.
+    Revoked,
+}
+
+/// The bounded, read-only AEC identity validator. Fold the live graph in with
+/// [`AecValidator::observe`] once per recompute; read the result with
+/// [`AecValidator::state`], [`AecValidator::fan_out_permitted`], and
+/// [`AecValidator::validated_module_id`].
+#[derive(Clone, Debug)]
+pub struct AecValidator {
+    /// The index to validate. `None` iff [`AecConfig::Off`] (state stays
+    /// [`AecState::NotConfigured`] forever).
+    target: Option<u64>,
+    state: AecState,
+    /// The `Validating → Failed` budget, applied *after* the deadline is armed.
+    timeout: Millis,
+    /// The absolute `Failed` deadline, armed the first time the graph reports
+    /// ready (the "registry sync barrier" of v3.4 §5.3) and never re-armed —
+    /// `graph_ready` is dynamic and can flap, but the epoch budget must not
+    /// restart. `None` until then: while the initial enumeration is still in
+    /// flight, a not-yet-seen index is *unknown*, not *absent*, so it must not
+    /// time out to `Failed`.
+    deadline: Option<Millis>,
+}
+
+impl AecValidator {
+    /// `timeout` is the `Validating → Failed` budget, counted from the moment
+    /// the graph first becomes ready (not from construction). An `Off` config
+    /// starts (and stays) [`AecState::NotConfigured`].
+    pub fn new(config: AecConfig, timeout: Millis) -> Self {
+        match config {
+            AecConfig::Off => Self {
+                target: None,
+                state: AecState::NotConfigured,
+                timeout,
+                deadline: None,
+            },
+            AecConfig::PulseModule(index) => Self {
+                target: Some(index),
+                state: AecState::Validating,
+                timeout,
+                deadline: None,
+            },
+        }
+    }
+
+    pub fn state(&self) -> AecState {
+        self.state
+    }
+
+    /// The validated index to place in
+    /// [`ExclusionCtx::aec_module_id`](crate::host::taint::ExclusionCtx) —
+    /// `Some` **only** in [`AecState::Validated`]. `None` everywhere else,
+    /// including `NotConfigured` (no AEC ⇒ nothing to exclude) and the
+    /// fail-closed states (whose `None` must be paired with
+    /// [`Self::fan_out_permitted`] `== false`, i.e. no fan-out at all — *not*
+    /// a fan-out that merely skips AEC exclusion).
+    pub fn validated_module_id(&self) -> Option<u64> {
+        match self.state {
+            AecState::Validated => self.target,
+            _ => None,
+        }
+    }
+
+    /// Whether fan-out may proceed at all right now. True only in
+    /// [`AecState::NotConfigured`] (fan out, no exclusion) and
+    /// [`AecState::Validated`] (fan out, excluding the identity). `Validating`,
+    /// `Failed` and `Revoked` all forbid it — silence over echo.
+    pub fn fan_out_permitted(&self) -> bool {
+        matches!(self.state, AecState::NotConfigured | AecState::Validated)
+    }
+
+    /// Fold one recompute's view of the graph into the machine.
+    ///
+    /// `graph_ready` is the observer's dynamic readiness
+    /// ([`Projection::graph_ready`](crate::host::observer::Projection)); `now`
+    /// is a monotonic millisecond clock. Positive evidence (a node bearing the
+    /// index) is authoritative and validates regardless of `graph_ready` —
+    /// seeing the node *is* seeing it — but the `Failed` deadline only begins
+    /// once `graph_ready` has first become true, so a slow initial enumeration
+    /// can never masquerade as a genuinely-absent module.
+    pub fn observe(&mut self, snapshot: &GraphSnapshot, graph_ready: bool, now: Millis) {
+        // `Off` (NotConfigured) and both sticky terminals are no-ops: there is
+        // nothing to look for, and a reappearing reused index must not revive a
+        // Failed/Revoked epoch (v3.4 §5.2 correction 3).
+        let Some(target) = self.target else {
+            return;
+        };
+        match self.state {
+            AecState::Validating => {
+                // Presence is checked *before* the deadline on purpose: a
+                // demonstrably-present identity validates regardless of the
+                // clock, even if the node is first seen just past the deadline
+                // (Codex phase-4 review, finding 2). The deadline only bounds
+                // the wait for an identity that is never seen — seeing it, late
+                // or not, is ground truth that the module exists, and excluding
+                // a real echo leg is always the safe answer. (A `Failed` can
+                // still pre-empt this when a `Tick`-only observation crosses the
+                // deadline first; that only makes the machine *more* fail-closed,
+                // never less.)
+                if self.index_present(snapshot, target) {
+                    self.state = AecState::Validated;
+                    return;
+                }
+                // Arm the deadline once, on the first ready graph.
+                if self.deadline.is_none() && graph_ready {
+                    self.deadline = Some(now.saturating_add(self.timeout));
+                }
+                if self.deadline.is_some_and(|deadline| now >= deadline) {
+                    self.state = AecState::Failed;
+                }
+            }
+            AecState::Validated => {
+                // Revocation is the whole identity gone (no node bears the
+                // index), not one leg corking — see the module header.
+                //
+                // ⚠️ **Deliberately NOT gated on `graph_ready`** (Codex
+                // phase-4 review, findings 1 + 4). Two forces pull opposite
+                // ways and this is the resolution:
+                //
+                // - Gating revoke on readiness would avoid a *spurious* revoke
+                //   from a transient empty snapshot seen while the module is
+                //   still live. But for the AEC that transient does not exist:
+                //   its four nodes are two `Stream/*` legs plus a null-sink-like
+                //   virtual sink/source, none of which claim a `device.id`, so
+                //   the phase-3 observer never *withholds* them
+                //   (`observer::classify` withholds only device-claiming nodes).
+                //   `index_present` therefore goes false only on a genuine
+                //   `global_remove` of every leg — a real unload — and a real
+                //   unload *should* revoke.
+                // - Worse, gating on readiness would REOPEN the reused-index
+                //   alias trap: if an unload+reload (indices recycle, §5.2
+                //   correction 3) both complete inside one not-ready churn
+                //   window, the ready snapshot would already show the *new*
+                //   module's node and we would never observe the empty gap —
+                //   silently aliasing onto an unrelated module. Revoking the
+                //   instant the gap appears, ready or not, is what closes it.
+                //
+                // This correctness rests on the phase-5/6 integration contract:
+                // **one `observe` per graph event, no coalescing across a module
+                // lifetime boundary.** Under coalescing, the empty gap between an
+                // old unload and a reused-index reload can be skipped. The
+                // robust fix that would not depend on that contract is a
+                // serial-continuity / observer-generation signal (the AEC nodes'
+                // `object.serial`s are fresh across a reload even when the index
+                // is not) — owed to a later hardening round, not built here.
+                if !self.index_present(snapshot, target) {
+                    self.state = AecState::Revoked;
+                }
+            }
+            AecState::NotConfigured | AecState::Failed | AecState::Revoked => {}
+        }
+    }
+
+    /// Whether any node in the snapshot bears the target module index. The same
+    /// exact-`u64`-equality predicate the taint engine roots on
+    /// (`taint/mod.rs`), kept here so "is the identity live?" has one
+    /// definition.
+    fn index_present(&self, snapshot: &GraphSnapshot, target: u64) -> bool {
+        snapshot
+            .nodes()
+            .any(|node| node.props.pulse_module_id == Some(target))
+    }
+}

src/host/aec/tests.rs

@@ -0,0 +1,374 @@
+//! Phase 4 exit gate (impl plan §4): a fake-clock / event-sequence transition
+//! matrix, because these are timing semantics a live poke cannot cover.
+
+use super::*;
+use crate::host::taint::snapshot::{
+    GlobalId, GraphSnapshot, MediaRole, NodeProps, NodeSnapshot, Serial,
+};
+
+/// A `Stream/Output/Audio` node carrying `pulse.module.id == module` (or none).
+/// Only the fields the validator reads matter; the rest take their defaults.
+fn node(serial: u64, module: Option<u64>) -> NodeSnapshot {
+    NodeSnapshot {
+        serial: Serial(serial),
+        id: GlobalId(serial as u32),
+        name: None,
+        role: MediaRole::StreamOutput,
+        props: NodeProps {
+            pulse_module_id: module,
+            ..NodeProps::default()
+        },
+    }
+}
+
+/// A snapshot holding exactly the given nodes (no ports/links/clients — the
+/// validator reads only nodes).
+fn snapshot(nodes: Vec<NodeSnapshot>) -> GraphSnapshot {
+    GraphSnapshot::new(nodes, vec![], vec![], vec![])
+}
+
+fn empty() -> GraphSnapshot {
+    snapshot(vec![])
+}
+
+const IDX: u64 = 536_870_919; // 0x20000007 — a real pipewire-pulse module index.
+const TIMEOUT: Millis = 2_000;
+
+// ---------------------------------------------------------------------------
+// Parsing (D5): off / pulse-module:<idx> / > u32::MAX / absent / malformed.
+// ---------------------------------------------------------------------------
+
+#[test]
+fn parses_off() {
+    assert_eq!(parse_aec_arg("off"), Ok(AecConfig::Off));
+}
+
+#[test]
+fn parses_pulse_module_index() {
+    assert_eq!(
+        parse_aec_arg("pulse-module:536870919"),
+        Ok(AecConfig::PulseModule(536_870_919)),
+    );
+}
+
+#[test]
+fn parses_index_beyond_u32() {
+    // v3.4 §5.2: compare as u64, never u32. A value one past u32::MAX must
+    // round-trip, not truncate or reject.
+    let big = u64::from(u32::MAX) + 1;
+    assert_eq!(
+        parse_aec_arg(&format!("pulse-module:{big}")),
+        Ok(AecConfig::PulseModule(big)),
+    );
+    assert_eq!(
+        parse_aec_arg(&format!("pulse-module:{}", u64::MAX)),
+        Ok(AecConfig::PulseModule(u64::MAX)),
+    );
+}
+
+#[test]
+fn rejects_empty() {
+    assert_eq!(parse_aec_arg(""), Err(AecParseError::Empty));
+}
+
+#[test]
+fn rejects_unknown_form() {
+    assert_eq!(parse_aec_arg("on"), Err(AecParseError::UnknownForm));
+    assert_eq!(parse_aec_arg("module:5"), Err(AecParseError::UnknownForm));
+    assert_eq!(parse_aec_arg("536870919"), Err(AecParseError::UnknownForm));
+}
+
+#[test]
+fn rejects_missing_index() {
+    assert_eq!(
+        parse_aec_arg("pulse-module:"),
+        Err(AecParseError::MissingIndex),
+    );
+}
+
+#[test]
+fn rejects_malformed_index() {
+    for bad in [
+        "pulse-module:-1",                   // sign
+        "pulse-module:+7",                   // sign
+        "pulse-module: 7",                   // leading whitespace
+        "pulse-module:7 ",                   // trailing whitespace
+        "pulse-module:0x7",                  // hex
+        "pulse-module:7.0",                  // non-integer
+        "pulse-module:abc",                  // non-numeric
+        "pulse-module:18446744073709551616", // u64::MAX + 1 (overflow)
+    ] {
+        assert_eq!(
+            parse_aec_arg(bad),
+            Err(AecParseError::InvalidIndex),
+            "{bad} should be InvalidIndex",
+        );
+    }
+}
+
+// ---------------------------------------------------------------------------
+// NotConfigured (--aec=off): benign, terminal, fan-out with no exclusion.
+// ---------------------------------------------------------------------------
+
+#[test]
+fn off_is_not_configured_and_permits_fan_out_with_no_identity() {
+    let mut v = AecValidator::new(AecConfig::Off, TIMEOUT);
+    assert_eq!(v.state(), AecState::NotConfigured);
+    assert!(v.fan_out_permitted());
+    assert_eq!(v.validated_module_id(), None);
+
+    // Even a snapshot full of module nodes never moves it off NotConfigured.
+    v.observe(&snapshot(vec![node(1, Some(IDX))]), true, 10_000);
+    assert_eq!(v.state(), AecState::NotConfigured);
+    assert!(v.fan_out_permitted());
+    assert_eq!(v.validated_module_id(), None);
+}
+
+// ---------------------------------------------------------------------------
+// Row: Validating → Validated on first matching node; no fan-out before.
+// ---------------------------------------------------------------------------
+
+#[test]
+fn validating_forbids_fan_out_and_exposes_no_identity() {
+    let v = AecValidator::new(AecConfig::PulseModule(IDX), TIMEOUT);
+    assert_eq!(v.state(), AecState::Validating);
+    assert!(!v.fan_out_permitted());
+    assert_eq!(v.validated_module_id(), None);
+}
+
+#[test]
+fn validating_to_validated_on_first_matching_node() {
+    let mut v = AecValidator::new(AecConfig::PulseModule(IDX), TIMEOUT);
+    // A node with a *different* index does not validate.
+    v.observe(&snapshot(vec![node(1, Some(IDX + 1))]), true, 0);
+    assert_eq!(v.state(), AecState::Validating);
+
+    v.observe(&snapshot(vec![node(2, Some(IDX))]), true, 100);
+    assert_eq!(v.state(), AecState::Validated);
+    assert!(v.fan_out_permitted());
+    assert_eq!(v.validated_module_id(), Some(IDX));
+}
+
+#[test]
+fn positive_evidence_validates_even_before_graph_ready() {
+    // Seeing the node is authoritative; readiness only gates the Failed clock.
+    let mut v = AecValidator::new(AecConfig::PulseModule(IDX), TIMEOUT);
+    v.observe(&snapshot(vec![node(1, Some(IDX))]), false, 0);
+    assert_eq!(v.state(), AecState::Validated);
+    assert_eq!(v.validated_module_id(), Some(IDX));
+}
+
+#[test]
+fn validated_index_is_compared_beyond_u32() {
+    let big = u64::from(u32::MAX) + 7;
+    let mut v = AecValidator::new(AecConfig::PulseModule(big), TIMEOUT);
+    // A node whose id equals `big` only in its low 32 bits must not match.
+    v.observe(
+        &snapshot(vec![node(1, Some(big & u64::from(u32::MAX)))]),
+        true,
+        0,
+    );
+    assert_eq!(v.state(), AecState::Validating);
+
+    v.observe(&snapshot(vec![node(2, Some(big))]), true, 1);
+    assert_eq!(v.state(), AecState::Validated);
+    assert_eq!(v.validated_module_id(), Some(big));
+}
+
+// ---------------------------------------------------------------------------
+// Row: Validating → Failed on deadline expiry; and the deadline is armed only
+// once the graph is ready (the registry sync barrier).
+// ---------------------------------------------------------------------------
+
+#[test]
+fn validating_to_failed_on_deadline_expiry() {
+    let mut v = AecValidator::new(AecConfig::PulseModule(IDX), TIMEOUT);
+    v.observe(&empty(), true, 0); // arms deadline at 0 + 2000
+    assert_eq!(v.state(), AecState::Validating);
+
+    v.observe(&empty(), true, TIMEOUT - 1);
+    assert_eq!(v.state(), AecState::Validating);
+
+    v.observe(&empty(), true, TIMEOUT); // now >= deadline
+    assert_eq!(v.state(), AecState::Failed);
+    assert!(!v.fan_out_permitted());
+    assert_eq!(v.validated_module_id(), None);
+}
+
+#[test]
+fn deadline_is_not_armed_until_graph_ready() {
+    // The whole point of arming-on-ready: a slow initial enumeration is
+    // "unknown", not "absent", and must never time out to Failed.
+    let mut v = AecValidator::new(AecConfig::PulseModule(IDX), TIMEOUT);
+    // Long past the would-be deadline, but the graph has never been ready.
+    v.observe(&empty(), false, 10 * TIMEOUT);
+    assert_eq!(v.state(), AecState::Validating);
+    // Still no Failed even much later, as long as ready stays false.
+    v.observe(&empty(), false, 100 * TIMEOUT);
+    assert_eq!(v.state(), AecState::Validating);
+
+    // And when readiness finally arrives, the FULL budget starts *there*, not
+    // relative to construction (Codex phase-4 review, finding 3): a mutant that
+    // armed a construction-relative deadline would fail immediately here.
+    let late = 200_000;
+    v.observe(&empty(), true, late); // first ready → arm at `late`
+    assert_eq!(v.state(), AecState::Validating);
+    v.observe(&empty(), true, late + TIMEOUT - 1);
+    assert_eq!(v.state(), AecState::Validating);
+    v.observe(&empty(), true, late + TIMEOUT);
+    assert_eq!(v.state(), AecState::Failed);
+}
+
+#[test]
+fn late_positive_evidence_wins_over_expired_deadline() {
+    // A node first seen just past the deadline still validates: the deadline
+    // only bounds the wait for an identity that is never seen, and a
+    // demonstrably-present module is ground truth (Codex phase-4 review,
+    // finding 2). Reachable only when the first post-deadline observation
+    // carries the node with no intervening Tick-only observation.
+    let mut v = AecValidator::new(AecConfig::PulseModule(IDX), TIMEOUT);
+    v.observe(&empty(), true, 0); // arm deadline at 2000
+    v.observe(&snapshot(vec![node(1, Some(IDX))]), true, TIMEOUT + 1);
+    assert_eq!(v.state(), AecState::Validated);
+    assert_eq!(v.validated_module_id(), Some(IDX));
+
+    // Whereas a Tick-only observation that crosses the deadline first pre-empts
+    // it to Failed (stickily), even if the node then shows up — fail-closed.
+    let mut w = AecValidator::new(AecConfig::PulseModule(IDX), TIMEOUT);
+    w.observe(&empty(), true, 0);
+    w.observe(&empty(), true, TIMEOUT); // Tick-only crosses the line first
+    assert_eq!(w.state(), AecState::Failed);
+    w.observe(&snapshot(vec![node(1, Some(IDX))]), true, TIMEOUT + 1);
+    assert_eq!(w.state(), AecState::Failed);
+}
+
+#[test]
+fn revokes_on_empty_even_while_not_ready() {
+    // Revocation is deliberately NOT gated on graph_ready (Codex phase-4 review,
+    // findings 1 + 4): the instant every node bearing the index is gone we
+    // revoke, ready or not, because gating on readiness would let an
+    // unload+reload that reused the index inside one not-ready churn window
+    // silently alias onto the new module. A mutant adding `&& graph_ready` to
+    // the revoke guard survives every other test but dies here.
+    let mut v = AecValidator::new(AecConfig::PulseModule(IDX), TIMEOUT);
+    v.observe(&snapshot(vec![node(1, Some(IDX))]), true, 0);
+    assert_eq!(v.state(), AecState::Validated);
+    v.observe(&empty(), false, 10); // identity gone during not-ready churn
+    assert_eq!(v.state(), AecState::Revoked);
+    assert!(!v.fan_out_permitted());
+}
+
+#[test]
+fn deadline_armed_once_survives_ready_flapping() {
+    // graph_ready is dynamic (it drops back to false while a Link is binding).
+    // The epoch budget must be armed on the *first* ready and not restarted.
+    let mut v = AecValidator::new(AecConfig::PulseModule(IDX), TIMEOUT);
+    v.observe(&empty(), true, 1_000); // arm at 1000 → deadline 3000
+    v.observe(&empty(), false, 2_000); // ready flaps off; must not disarm
+    assert_eq!(v.state(), AecState::Validating);
+    // At the original deadline it fails, even though ready is false now — the
+    // budget did not restart from the flap.
+    v.observe(&empty(), false, 3_000);
+    assert_eq!(v.state(), AecState::Failed);
+}
+
+#[test]
+fn failed_is_sticky_even_if_the_index_reappears() {
+    let mut v = AecValidator::new(AecConfig::PulseModule(IDX), TIMEOUT);
+    v.observe(&empty(), true, 0);
+    v.observe(&empty(), true, TIMEOUT);
+    assert_eq!(v.state(), AecState::Failed);
+
+    // A node bearing the index shows up late — must not resurrect the epoch.
+    v.observe(&snapshot(vec![node(1, Some(IDX))]), true, TIMEOUT + 1);
+    assert_eq!(v.state(), AecState::Failed);
+    assert!(!v.fan_out_permitted());
+    assert_eq!(v.validated_module_id(), None);
+}
+
+// ---------------------------------------------------------------------------
+// Row: partial-node disappearance ⇒ stays Validated; all gone ⇒ Revoked.
+// ---------------------------------------------------------------------------
+
+#[test]
+fn partial_leg_disappearance_stays_validated() {
+    let mut v = AecValidator::new(AecConfig::PulseModule(IDX), TIMEOUT);
+    // The module's four nodes all carry the index.
+    let four = snapshot(vec![
+        node(1, Some(IDX)),
+        node(2, Some(IDX)),
+        node(3, Some(IDX)),
+        node(4, Some(IDX)),
+    ]);
+    v.observe(&four, true, 0);
+    assert_eq!(v.state(), AecState::Validated);
+
+    // Three legs cork/relink away; one still bears the index → still Validated.
+    v.observe(&snapshot(vec![node(4, Some(IDX))]), true, 10);
+    assert_eq!(v.state(), AecState::Validated);
+    assert_eq!(v.validated_module_id(), Some(IDX));
+}
+
+#[test]
+fn all_nodes_gone_revokes() {
+    let mut v = AecValidator::new(AecConfig::PulseModule(IDX), TIMEOUT);
+    v.observe(&snapshot(vec![node(1, Some(IDX))]), true, 0);
+    assert_eq!(v.state(), AecState::Validated);
+
+    // The whole identity unloads: no node bears the index any more.
+    v.observe(&empty(), true, 10);
+    assert_eq!(v.state(), AecState::Revoked);
+}
+
+#[test]
+fn revoked_stops_fan_out_and_exposes_no_identity() {
+    let mut v = AecValidator::new(AecConfig::PulseModule(IDX), TIMEOUT);
+    v.observe(&snapshot(vec![node(1, Some(IDX))]), true, 0);
+    v.observe(&empty(), true, 10);
+    assert_eq!(v.state(), AecState::Revoked);
+    assert!(!v.fan_out_permitted());
+    assert_eq!(v.validated_module_id(), None);
+}
+
+#[test]
+fn a_node_that_merely_changes_index_revokes() {
+    // Not a disappearance in the id sense, but the *identity* is gone: no node
+    // bears our index any more, even though a same-serial node lingers.
+    let mut v = AecValidator::new(AecConfig::PulseModule(IDX), TIMEOUT);
+    v.observe(&snapshot(vec![node(1, Some(IDX))]), true, 0);
+    v.observe(&snapshot(vec![node(1, Some(IDX + 1))]), true, 10);
+    assert_eq!(v.state(), AecState::Revoked);
+}
+
+// ---------------------------------------------------------------------------
+// Row: a retained stale index does not alias onto a reloaded module — indices
+// ARE reused (v3.4 §5.2 correction 3). This is the sharpest safety property.
+// ---------------------------------------------------------------------------
+
+#[test]
+fn revoked_index_does_not_alias_onto_a_reloaded_module() {
+    let mut v = AecValidator::new(AecConfig::PulseModule(IDX), TIMEOUT);
+    v.observe(&snapshot(vec![node(1, Some(IDX))]), true, 0);
+    v.observe(&empty(), true, 10);
+    assert_eq!(v.state(), AecState::Revoked);
+
+    // A *different* module later reloads and pactl hands it the very same
+    // index (measured: 536870919 came back verbatim). A resurrecting machine
+    // would silently start excluding this unrelated module's node. Ours must
+    // stay Revoked and fail closed; a real reload gets a fresh validator.
+    v.observe(&snapshot(vec![node(99, Some(IDX))]), true, 20);
+    assert_eq!(v.state(), AecState::Revoked);
+    assert!(!v.fan_out_permitted());
+    assert_eq!(v.validated_module_id(), None);
+}
+
+#[test]
+fn a_fresh_validator_re_validates_the_reused_index() {
+    // The counterpart: because peerspeak re-tells pixelpass the index on every
+    // load, the correct response to a reload is a new machine, which validates
+    // the reused index cleanly — proving stickiness costs nothing legitimate.
+    let mut v = AecValidator::new(AecConfig::PulseModule(IDX), TIMEOUT);
+    v.observe(&snapshot(vec![node(1, Some(IDX))]), true, 0);
+    assert_eq!(v.state(), AecState::Validated);
+    assert_eq!(v.validated_module_id(), Some(IDX));
+}

src/host/audio.rs

@@ -365,6 +365,9 @@ fn load_module(args: &[&str]) -> Result<u32> {
         .context("pactl returned non-UTF-8")?
         .trim()
         .to_string();
+    // Genuinely 32-bit, unlike `object.serial`: this is a PulseAudio module
+    // index (`pa_module.index`, `uint32_t`), which `pactl unload-module` takes
+    // back verbatim. Do not widen it.
     id_str
         .parse::<u32>()
         .with_context(|| format!("pactl returned unexpected module ID: {id_str:?}"))
@@ -532,13 +535,21 @@ fn run_router(
                         return;
                     };
                     if props.get("node.name") == Some(sink_name_owned.as_str()) {
-                        if let Some(serial) = props
-                            .get("object.serial")
-                            .and_then(|s| s.parse::<u32>().ok())
-                        {
-                            state_for_reg.borrow_mut().sink_serial = Some(serial);
-                            tracing::info!(serial, "audio routing: pixelpass sink registered");
-                            try_flush(&state_for_reg, &event_tx_for_reg);
+                        match props.get("object.serial").and_then(parse_object_serial) {
+                            Some(serial) => {
+                                state_for_reg.borrow_mut().sink_serial = Some(serial);
+                                tracing::info!(serial, "audio routing: pixelpass sink registered");
+                                try_flush(&state_for_reg, &event_tx_for_reg);
+                            }
+                            // Never silently: without a serial `try_flush` can
+                            // never route anything, so the whole app-filter mode
+                            // is dead and the only symptom is missing audio.
+                            None => tracing::warn!(
+                                node_id = obj.id,
+                                serial = props.get("object.serial").unwrap_or("<absent>"),
+                                "audio routing: pixelpass sink has no usable object.serial; \
+                                 stream rerouting disabled"
+                            ),
                         }
                         return;
                     }
@@ -591,8 +602,30 @@ fn run_router(
     Ok(())
 }
 
+/// Parse a PipeWire `object.serial` property value.
+///
+/// `object.serial` is a **64-bit** monotonically-increasing counter
+/// (`pw_global`'s serial is `uint64_t`); it is *not* a `pw` object id
+/// (those are `u32` and get recycled — the serial exists precisely so
+/// that recycled ids can be disambiguated). Parsing it as `u32` silently
+/// yields `None` past `u32::MAX`, which on a long-lived daemon means the
+/// sink is never registered and no stream is ever routed.
+///
+/// Strict on purpose: PipeWire emits a bare decimal, so anything else
+/// (empty, signed, whitespace-padded, non-numeric, overflowing) is a
+/// property we do not understand and must not guess at. Leading zeroes
+/// are accepted — they are unambiguous and parse to the same value.
+pub(crate) fn parse_object_serial(raw: &str) -> Option<u64> {
+    if raw.is_empty() || !raw.bytes().all(|b| b.is_ascii_digit()) {
+        return None;
+    }
+    raw.parse::<u64>().ok()
+}
+
 struct RouterState {
-    sink_serial: Option<u32>,
+    /// See [`parse_object_serial`] — 64-bit, and not interchangeable with
+    /// the `u32` node ids in `routed_node_ids` / `pending`.
+    sink_serial: Option<u64>,
     default_metadata: Option<pipewire::metadata::Metadata>,
     routed_node_ids: Vec<u32>,
     pending: Vec<u32>,
@@ -654,3 +687,79 @@ fn try_flush(
         let _ = event_tx.send(Event::FirstRoutedStream);
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn object_serial_parses_past_u32() {
+        // The regression this fix exists for: a serial one past `u32::MAX`
+        // used to parse as `None` and silently disable rerouting.
+        let beyond = u64::from(u32::MAX) + 1;
+        assert_eq!(parse_object_serial(&beyond.to_string()), Some(beyond));
+        assert_eq!(
+            parse_object_serial(&u64::MAX.to_string()),
+            Some(u64::MAX),
+            "the full 64-bit range must round-trip"
+        );
+    }
+
+    #[test]
+    fn object_serial_accepts_ordinary_serials() {
+        // Without this the valid cases are only 1, 10 and 20 digits long, and
+        // a length-gated mutant (`if (2..10).contains(&raw.len()) { None }`)
+        // survives the whole suite while rejecting every serial a freshly
+        // started daemon actually hands out. (Codex, round 1.)
+        for serial in 0_u64..=1024 {
+            assert_eq!(parse_object_serial(&serial.to_string()), Some(serial));
+        }
+        assert_eq!(parse_object_serial("123456789"), Some(123_456_789));
+        assert_eq!(
+            parse_object_serial("007"),
+            Some(7),
+            "leading zeroes are fine"
+        );
+    }
+
+    #[test]
+    fn object_serial_boundary_values() {
+        assert_eq!(parse_object_serial("0"), Some(0));
+        assert_eq!(parse_object_serial("1"), Some(1));
+        let max32 = u64::from(u32::MAX);
+        assert_eq!(parse_object_serial(&max32.to_string()), Some(max32));
+        assert_eq!(
+            parse_object_serial(&(max32 - 1).to_string()),
+            Some(max32 - 1)
+        );
+    }
+
+    #[test]
+    fn object_serial_round_trips_through_the_metadata_string() {
+        // `try_flush` writes the serial back out as a decimal string for
+        // `target.object`; widening must not introduce a formatting change.
+        for raw in ["0", "4294967296", "18446744073709551615"] {
+            let parsed = parse_object_serial(raw).expect("valid serial");
+            assert_eq!(parsed.to_string(), raw);
+        }
+    }
+
+    #[test]
+    fn object_serial_rejects_malformed() {
+        for raw in [
+            "",
+            " 12",
+            "12 ",
+            "+12",
+            "-1",
+            "1.0",
+            "0x10",
+            "12a",
+            "abc",
+            // u64::MAX + 1 — overflow must be rejected, not wrapped.
+            "18446744073709551616",
+        ] {
+            assert_eq!(parse_object_serial(raw), None, "should reject {raw:?}");
+        }
+    }
+}

src/host/audit/metrics.rs

@@ -0,0 +1,269 @@
+//! O5 measurement, pure (impl plan §5.2).
+//!
+//! v3.4 §6.4 asserts "a full recompute per graph event is fine for v1". The
+//! impl plan closes O5 by refusing to let that rest on a node count: what has
+//! to be recorded is the **graph-event rate**, the **recompute duration
+//! distribution and maximum**, and **whether events queue behind recompute or
+//! logging**.
+//!
+//! Everything here is arithmetic over samples the caller supplies. The clock
+//! reads live at the I/O edge ([`super::sink`]), which is what keeps the
+//! statistics unit-testable: a test feeds a hand-written sample sequence and
+//! asserts the summary exactly, with no timing flake.
+//!
+//! **The queueing measure is a proxy, and a one-directional one.** libpipewire
+//! dispatches registry callbacks serially on its own loop thread and exposes no
+//! queue depth, so nothing here can read a backlog directly. What it can see is
+//! that the observer thread was *continuously busy*: if an event begins being
+//! handled within [`QUEUE_THRESHOLD_US`] of the previous sample's completion,
+//! it was almost certainly already waiting while that recompute ran. That makes
+//! [`Summary::queued_events`] a **lower bound** — a genuine backlog always shows
+//! up in it, but a burst that happens to arrive exactly as the loop goes idle is
+//! counted as un-queued. Combined with [`Summary::busy_fraction`] (which needs
+//! no inference at all) it is enough to answer O5 in the direction that matters:
+//! a low busy fraction with zero queued events is headroom, and anything else is
+//! a number to argue about rather than an assumption to inherit.
+
+use serde::Serialize;
+
+use crate::host::observer::EventKind;
+
+/// An event beginning this close behind the previous sample's completion is
+/// counted as having queued. Deliberately tight: the cost of being wrong in the
+/// generous direction is a metric that overstates backlog and sends a later
+/// round chasing a non-problem.
+pub const QUEUE_THRESHOLD_US: u64 = 100;
+
+/// Upper bounds of the duration histogram, microseconds. A twelfth (overflow)
+/// bucket catches everything at or above the last bound. Log-ish spacing: the
+/// interesting question is which order of magnitude a recompute lands in, not
+/// its exact microsecond.
+pub const BUCKET_BOUNDS_US: [u64; 11] = [
+    50, 100, 250, 500, 1_000, 2_500, 5_000, 10_000, 25_000, 50_000, 100_000,
+];
+
+/// Human labels for the histogram buckets, parallel to [`BUCKET_BOUNDS_US`]
+/// plus the overflow bucket.
+pub const BUCKET_LABELS: [&str; 12] = [
+    "<50us", "<100us", "<250us", "<500us", "<1ms", "<2.5ms", "<5ms", "<10ms", "<25ms", "<50ms",
+    "<100ms", ">=100ms",
+];
+
+/// A bucketed duration distribution with exact count, sum and maximum.
+///
+/// Bounded memory by construction — the audit runs for as long as a share does,
+/// and keeping every sample to compute an exact percentile would grow without
+/// limit. The maximum, which is the number O5 actually cares about, is kept
+/// exactly; percentiles are reported as the bucket they fall in.
+#[derive(Clone, Debug, Default, PartialEq, Eq)]
+pub struct Histogram {
+    buckets: [u64; 12],
+    count: u64,
+    sum_us: u64,
+    max_us: u64,
+}
+
+impl Histogram {
+    pub fn record(&mut self, us: u64) {
+        let index = BUCKET_BOUNDS_US
+            .iter()
+            .position(|&bound| us < bound)
+            .unwrap_or(BUCKET_BOUNDS_US.len());
+        self.buckets[index] += 1;
+        self.count += 1;
+        self.sum_us = self.sum_us.saturating_add(us);
+        self.max_us = self.max_us.max(us);
+    }
+
+    pub fn count(&self) -> u64 {
+        self.count
+    }
+
+    pub fn max_us(&self) -> u64 {
+        self.max_us
+    }
+
+    pub fn sum_us(&self) -> u64 {
+        self.sum_us
+    }
+
+    pub fn mean_us(&self) -> Option<u64> {
+        (self.count > 0).then(|| self.sum_us / self.count)
+    }
+
+    /// The label of the bucket the `q`-quantile falls in (`q` in `0.0..=1.0`),
+    /// or `None` when nothing has been recorded.
+    ///
+    /// Uses the *nearest-rank* definition: the bucket containing the
+    /// `ceil(q · count)`-th sample in ascending order. Reported as a bucket
+    /// rather than a number because interpolating inside a bucket would invent
+    /// precision the histogram does not have.
+    pub fn quantile_bucket(&self, q: f64) -> Option<&'static str> {
+        if self.count == 0 {
+            return None;
+        }
+        let q = q.clamp(0.0, 1.0);
+        // Rank is 1-based; q = 0 still names the bucket holding the smallest
+        // sample rather than degenerating to "no samples".
+        let rank = ((q * self.count as f64).ceil() as u64).max(1);
+        let mut cumulative = 0u64;
+        for (index, &n) in self.buckets.iter().enumerate() {
+            cumulative += n;
+            if cumulative >= rank {
+                return Some(BUCKET_LABELS[index]);
+            }
+        }
+        // Unreachable while `count` is the sum of the buckets, but returning the
+        // top bucket is the fail-loud answer rather than a panic in a metric.
+        Some(BUCKET_LABELS[BUCKET_LABELS.len() - 1])
+    }
+
+    /// Non-empty buckets as `(label, count)`, ascending. Empty buckets are
+    /// dropped so a summary line stays readable.
+    pub fn distribution(&self) -> Vec<(&'static str, u64)> {
+        self.buckets
+            .iter()
+            .enumerate()
+            .filter(|&(_, &n)| n > 0)
+            .map(|(index, &n)| (BUCKET_LABELS[index], n))
+            .collect()
+    }
+}
+
+/// One handled event, as timed by the I/O edge.
+///
+/// Ticks are the AEC validator's clock, not graph changes, so [`Metrics`] counts
+/// them separately — folding them into the event rate would inflate it by a
+/// constant 4 Hz and hide the real graph churn.
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+pub struct Sample {
+    /// Monotonic microseconds (since observer start) at which handling began.
+    pub at_us: u64,
+    /// Microseconds between the previous sample's completion and `at_us`. Zero
+    /// for the first sample.
+    pub gap_us: u64,
+    /// Time spent in the AEC observe + taint recompute.
+    pub recompute_us: u64,
+    /// Time spent serialising and writing the record, zero when nothing was
+    /// emitted. Separate from `recompute_us` because O5 asks about queueing
+    /// behind recompute **or logging** — and if logging turns out to dominate,
+    /// that is a fixable problem of a different kind.
+    pub emit_us: u64,
+    pub kind: EventKind,
+}
+
+/// Rolling O5 state. Fold samples in with [`Metrics::record`]; read with
+/// [`Metrics::summary`].
+#[derive(Clone, Debug, Default)]
+pub struct Metrics {
+    graph_events: u64,
+    tick_events: u64,
+    emitted_records: u64,
+    recompute: Histogram,
+    emit: Histogram,
+    busy_us: u64,
+    queued_events: u64,
+    first_event_us: Option<u64>,
+    last_completion_us: u64,
+}
+
+impl Metrics {
+    pub fn record(&mut self, sample: Sample) {
+        match sample.kind {
+            EventKind::Graph => self.graph_events += 1,
+            EventKind::Tick => self.tick_events += 1,
+        }
+        self.recompute.record(sample.recompute_us);
+        if sample.emit_us > 0 {
+            self.emitted_records += 1;
+            self.emit.record(sample.emit_us);
+        }
+        self.busy_us = self
+            .busy_us
+            .saturating_add(sample.recompute_us)
+            .saturating_add(sample.emit_us);
+
+        // The first sample has no predecessor to have queued behind.
+        if self.first_event_us.is_some() && sample.gap_us <= QUEUE_THRESHOLD_US {
+            self.queued_events += 1;
+        }
+        self.first_event_us.get_or_insert(sample.at_us);
+        self.last_completion_us = sample
+            .at_us
+            .saturating_add(sample.recompute_us)
+            .saturating_add(sample.emit_us);
+    }
+
+    pub fn summary(&self) -> Summary {
+        let span_us = self
+            .first_event_us
+            .map(|first| self.last_completion_us.saturating_sub(first))
+            .unwrap_or(0);
+        // A rate needs a span to divide by; one event in zero elapsed time has
+        // no rate, and reporting a made-up one is worse than reporting none.
+        let graph_events_per_sec = (span_us > 0)
+            .then(|| self.graph_events as f64 * 1_000_000.0 / span_us as f64)
+            .map(round_2);
+        let busy_fraction = (span_us > 0).then(|| round_4(self.busy_us as f64 / span_us as f64));
+
+        Summary {
+            graph_events: self.graph_events,
+            tick_events: self.tick_events,
+            emitted_records: self.emitted_records,
+            span_us,
+            graph_events_per_sec,
+            recompute_max_us: self.recompute.max_us(),
+            recompute_mean_us: self.recompute.mean_us(),
+            recompute_p50: self.recompute.quantile_bucket(0.50),
+            recompute_p90: self.recompute.quantile_bucket(0.90),
+            recompute_p99: self.recompute.quantile_bucket(0.99),
+            recompute_distribution: self.recompute.distribution(),
+            emit_max_us: self.emit.max_us(),
+            emit_mean_us: self.emit.mean_us(),
+            emit_distribution: self.emit.distribution(),
+            busy_us: self.busy_us,
+            busy_fraction,
+            queued_events: self.queued_events,
+            queue_threshold_us: QUEUE_THRESHOLD_US,
+        }
+    }
+}
+
+/// The O5 answer, as emitted.
+#[derive(Clone, Debug, PartialEq, Serialize)]
+pub struct Summary {
+    pub graph_events: u64,
+    pub tick_events: u64,
+    pub emitted_records: u64,
+    /// First event to last completion, microseconds.
+    pub span_us: u64,
+    pub graph_events_per_sec: Option<f64>,
+    pub recompute_max_us: u64,
+    pub recompute_mean_us: Option<u64>,
+    pub recompute_p50: Option<&'static str>,
+    pub recompute_p90: Option<&'static str>,
+    pub recompute_p99: Option<&'static str>,
+    pub recompute_distribution: Vec<(&'static str, u64)>,
+    pub emit_max_us: u64,
+    pub emit_mean_us: Option<u64>,
+    pub emit_distribution: Vec<(&'static str, u64)>,
+    /// Total observer-thread time spent recomputing and logging.
+    pub busy_us: u64,
+    /// `busy_us / span_us` — the share of wall time the observer thread could
+    /// not be servicing PipeWire. Needs no inference, unlike `queued_events`.
+    pub busy_fraction: Option<f64>,
+    /// Events that began within `queue_threshold_us` of the previous sample's
+    /// completion — a **lower bound** on backlog, see the module header.
+    pub queued_events: u64,
+    pub queue_threshold_us: u64,
+}
+
+/// Keep the JSON readable: a rate to two decimals and a fraction to four are
+/// well past the precision any of this is good to.
+fn round_2(value: f64) -> f64 {
+    (value * 100.0).round() / 100.0
+}
+
+fn round_4(value: f64) -> f64 {
+    (value * 10_000.0).round() / 10_000.0
+}

src/host/audit/mod.rs

@@ -0,0 +1,487 @@
+//! Phase 5 — dry-run audit mode 🚦 (impl plan §5).
+//!
+//! **This phase adds no capability. Its entire purpose is to be wrong loudly
+//! and safely.** It runs phases 2–4 against the *live* graph on every graph
+//! event and reports what they conclude. It creates no links, loads no modules,
+//! and changes no routing — the only thing it produces is a line of JSON.
+//!
+//! Why this is the gate the plan marks 🚦: the defects that matter here are
+//! graph-*reasoning* defects. The 57 phase-2 fixture tests prove the engine
+//! matches my model of PipeWire; only a live run proves my model matches
+//! PipeWire. A wrong answer at this phase costs a log line. The same wrong
+//! answer in phase 6 costs an echo — the sharer's own voice, copied back into
+//! the share, which is the failure this whole design exists to prevent.
+//!
+//! ## The one structural requirement (§5.1)
+//!
+//! Every emitted record carries the **complete candidate universe partitioned
+//! into exact eligible and excluded sets**, with a stable reason code on each
+//! excluded row — never a spot check on named nodes. Checking only the nodes a
+//! row names constrains nothing about the rest, and it lets the degenerate
+//! "exclude everything" implementation pass: that build is silent, produces no
+//! echo, and satisfies any assertion phrased purely as *this must be excluded*.
+//! Asserting the eligible half of each row is what fails it. That requirement is
+//! also the plan's answer to open question O7 (over-exclusion needs no separate
+//! gate — it is subsumed by this one).
+//!
+//! ## What is deliberately *not* here
+//!
+//! - **No link creation, and no code path that could reach one.** The auditor
+//!   consumes a [`Projection`] and returns a record. It has no handle to
+//!   anything mutable.
+//! - **No stdout.** Records go to stderr as JSON Lines
+//!   ([`sink`]) because peerspeak parses pixelpass's stdout event stream
+//!   (`screenshare/mod.rs:92`); a stray line there corrupts it.
+//! - **No `--aec` CLI flag.** That surface is phase 7's mode selector. The audit
+//!   takes its AEC identity from `PIXELPASS_AUDIO_AUDIT_AEC` through the
+//!   *same* [`parse_aec_arg`] the real flag will use, so the parser and the
+//!   validator are both exercised without committing to a public interface
+//!   before it is designed.
+//!
+//! ## Fan-out gating vs. taint (read before interpreting a record)
+//!
+//! Two independent things can exclude a candidate and the record keeps them
+//! distinguishable:
+//!
+//! - The **taint engine** (phase 2) excludes individual nodes with its own
+//!   reason codes — `peerspeak-owned`, `aec-identity`, `tainted-upstream`, …
+//! - The **AEC validator** (phase 4) can forbid fan-out *entirely*, regardless
+//!   of taint, whenever the configured identity is unvalidated, failed or
+//!   revoked. Silence over echo.
+//!
+//! When the gate is shut, a candidate the engine would have called eligible is
+//! reported excluded with an audit-level reason ([`GateReason`]); a candidate
+//! the engine excluded on its own keeps *its* reason, because that names the
+//! mechanism that actually applies to it. `fan_out_permitted` on the record
+//! carries the gate state, so the two cases are always tellable apart.
+//!
+//! **Consequence for the §5.1 matrix:** every row whose point is the
+//! eligible/excluded partition must run with `PIXELPASS_AUDIO_AUDIT_AEC=off`
+//! (state `NotConfigured`, gate open). Row 12 — the AEC lifecycle row — is the
+//! one that runs with a real `pulse-module:<idx>`, and the gate slamming shut is
+//! precisely what it asserts.
+
+#![allow(dead_code)] // Trigger paths are wired by `sink` + `run`; rows are read by tests.
+
+pub mod metrics;
+pub mod run;
+pub mod sink;
+
+#[cfg(test)]
+mod tests;
+
+use serde::Serialize;
+
+use crate::host::aec::{AecConfig, AecState, AecValidator};
+use crate::host::observer::{EventKind, Millis, Projection, Readiness};
+use crate::host::taint::snapshot::Serial;
+use crate::host::taint::{Decisions, Eligibility, ExclusionCtx, StickyState, evaluate};
+
+/// How long the AEC validator may sit in `Validating` after the graph first
+/// reports ready before failing closed. Generous relative to the observer's own
+/// 2 s readiness budget: in the audit a `Failed` is a diagnostic, and timing out
+/// early would report an absent module that was merely slow to appear.
+pub const AEC_VALIDATION_TIMEOUT_MILLIS: Millis = 5_000;
+
+/// Everything the auditor needs beyond the live graph.
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+pub struct AuditConfig {
+    /// The AEC identity to validate, as parsed from
+    /// `PIXELPASS_AUDIO_AUDIT_AEC`. Defaults to [`AecConfig::Off`] — an audit
+    /// run is not a share, so "there is no echo canceller in play" is the
+    /// honest default, and it is what leaves the fan-out gate open for the
+    /// partition rows.
+    pub aec: AecConfig,
+    pub aec_timeout: Millis,
+}
+
+impl Default for AuditConfig {
+    fn default() -> Self {
+        Self {
+            aec: AecConfig::Off,
+            aec_timeout: AEC_VALIDATION_TIMEOUT_MILLIS,
+        }
+    }
+}
+
+/// An audit-level exclusion: the AEC validator has shut the fan-out gate. These
+/// codes are disjoint from the taint engine's
+/// [`Reason::code`](crate::host::taint::Reason::code) values, so a reader never
+/// has to know which layer produced a code to interpret it.
+// The shared `Aec` prefix is the point: `GateReason::Validating` and
+// `AecState::Validating` would be one careless glob import away from being
+// confused, and these three are the *audit's* view of that machine, not the
+// machine itself.
+#[allow(clippy::enum_variant_names)]
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+pub enum GateReason {
+    /// The configured AEC identity has not been seen yet. Not an error — the
+    /// module may still be loading — but no fan-out happens meanwhile.
+    AecValidating,
+    /// The deadline passed with the identity never observed.
+    AecFailed,
+    /// The whole identity disappeared mid-run: every node bearing the index is
+    /// gone (v3.4 §5.3).
+    AecRevoked,
+}
+
+impl GateReason {
+    pub fn code(self) -> &'static str {
+        match self {
+            Self::AecValidating => "aec-validating",
+            Self::AecFailed => "aec-failed",
+            Self::AecRevoked => "aec-revoked",
+        }
+    }
+
+    /// The gate reason implied by a validator state, or `None` when fan-out is
+    /// permitted. Mirrors [`AecValidator::fan_out_permitted`] — kept as one
+    /// `match` over the same enum so the two cannot drift: every state that
+    /// permits fan-out maps to `None` and every state that forbids it maps to a
+    /// code.
+    pub fn from_state(state: AecState) -> Option<Self> {
+        match state {
+            AecState::NotConfigured | AecState::Validated => None,
+            AecState::Validating => Some(Self::AecValidating),
+            AecState::Failed => Some(Self::AecFailed),
+            AecState::Revoked => Some(Self::AecRevoked),
+        }
+    }
+}
+
+/// Stable string for an [`AecState`], for the record's `aec_state` field.
+///
+/// Defined here rather than on [`AecState`] to keep the merged phase-4 module
+/// untouched by a reporting concern.
+fn aec_state_code(state: AecState) -> &'static str {
+    match state {
+        AecState::NotConfigured => "not-configured",
+        AecState::Validating => "validating",
+        AecState::Validated => "validated",
+        AecState::Failed => "failed",
+        AecState::Revoked => "revoked",
+    }
+}
+
+/// Stable string for the observer's readiness epoch.
+fn readiness_code(readiness: Readiness) -> &'static str {
+    match readiness {
+        Readiness::Waiting => "waiting",
+        Readiness::Complete => "complete",
+        Readiness::TimedOut => "timed-out",
+    }
+}
+
+/// One candidate node's effective answer. `reason` is `None` exactly when
+/// `eligible` is true.
+#[derive(Clone, Debug, PartialEq, Eq, Serialize)]
+pub struct AuditRow {
+    pub serial: u64,
+    pub name: Option<String>,
+    pub eligible: bool,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub reason: Option<&'static str>,
+    /// The exclusion was carried over from a previous snapshot rather than
+    /// derived from the current topology (phase-2 stickiness).
+    pub sticky: bool,
+}
+
+/// A tainted node of *any* media role, not just fan-out candidates. Candidates
+/// already appear in [`AuditBody::candidates`]; this is the diagnostic view —
+/// when a candidate's exclusion is a surprise, the taint that reached it is the
+/// next question, and it usually sits on a node that is not itself a candidate.
+#[derive(Clone, Debug, PartialEq, Eq, Serialize)]
+pub struct TaintRow {
+    pub serial: u64,
+    pub name: Option<String>,
+    pub reason: &'static str,
+    pub sticky: bool,
+}
+
+/// A node carrying a peerspeak ownership carrier on a role the engine does not
+/// honour it on (round 10, R10-1). `role` is the point of the row: it says
+/// which non-producer role the tag turned up on, which is what distinguishes a
+/// producer-side bug from an impersonation attempt.
+#[derive(Clone, Debug, PartialEq, Eq, Serialize)]
+pub struct IgnoredTagRow {
+    pub serial: u64,
+    pub name: Option<String>,
+    pub role: &'static str,
+}
+
+/// The decision content of one recompute — everything except which recompute it
+/// was. Split out from [`AuditRecord`] so "did anything actually change?" is a
+/// derived `==` rather than a hand-maintained field comparison that a later
+/// field addition could silently fall out of.
+#[derive(Clone, Debug, PartialEq, Eq, Serialize)]
+pub struct AuditBody {
+    /// The observer's dynamic readiness. False ⇒ every candidate is excluded
+    /// `graph-not-ready`; no decision from a partial graph is a decision.
+    pub graph_ready: bool,
+    /// The sticky readiness epoch, which distinguishes the three ways
+    /// `graph_ready` can be false (see [`Projection::readiness`]).
+    pub epoch: &'static str,
+    pub aec_state: &'static str,
+    /// The index handed to the taint engine — `Some` only while `Validated`.
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub aec_module_id: Option<u64>,
+    /// Whether the AEC validator permits fan-out at all right now.
+    pub fan_out_permitted: bool,
+    /// The audit-level reason fan-out is forbidden, when it is.
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub gate_reason: Option<&'static str>,
+    /// **The complete candidate universe**, ascending by serial — every
+    /// `Stream/Output/Audio` node in the snapshot, partitioned. §5.1's exact
+    /// partition is `candidates`, not a subset of it.
+    pub candidates: Vec<AuditRow>,
+    pub eligible_count: usize,
+    pub excluded_count: usize,
+    /// Taint across all node roles, ascending by serial.
+    pub taint: Vec<TaintRow>,
+    /// Nodes carrying a peerspeak ownership carrier that the engine
+    /// **ignored** because they are not `Stream/Output/Audio` (round 10,
+    /// R10-1). Normally empty; a non-empty list means either peerspeak is
+    /// tagging something it should not, or a process is impersonating the
+    /// tag. Neither is an exclusion, and neither should be silent.
+    ///
+    /// Omitted from the JSONL when empty, so it costs nothing on the common
+    /// path and is impossible to miss when it is not.
+    #[serde(skip_serializing_if = "Vec::is_empty")]
+    pub ignored_ownership_tags: Vec<IgnoredTagRow>,
+}
+
+impl AuditBody {
+    /// Serials of eligible candidates, ascending — the half of the partition an
+    /// exclude-everything build fails.
+    pub fn eligible(&self) -> Vec<u64> {
+        self.candidates
+            .iter()
+            .filter(|row| row.eligible)
+            .map(|row| row.serial)
+            .collect()
+    }
+
+    /// `(serial, reason code)` for excluded candidates, ascending.
+    pub fn excluded(&self) -> Vec<(u64, &'static str)> {
+        self.candidates
+            .iter()
+            .filter(|row| !row.eligible)
+            .map(|row| (row.serial, row.reason.unwrap_or("?")))
+            .collect()
+    }
+
+    /// The eligible candidate with this name, if any. Convenience for the
+    /// matrix rows, which name nodes rather than serials.
+    pub fn row_named(&self, name: &str) -> Option<&AuditRow> {
+        self.candidates
+            .iter()
+            .find(|row| row.name.as_deref() == Some(name))
+    }
+}
+
+/// One recompute, as emitted.
+#[derive(Clone, Debug, PartialEq, Eq, Serialize)]
+pub struct AuditRecord {
+    /// Monotonic per-run counter over *every* recompute, emitted or suppressed,
+    /// so a gap in the emitted sequence is visibly a suppression rather than a
+    /// lost line.
+    pub seq: u64,
+    pub trigger: &'static str,
+    /// Observer-clock milliseconds at which this recompute ran.
+    pub at_ms: Millis,
+    #[serde(flatten)]
+    pub body: AuditBody,
+}
+
+/// What one [`Auditor::observe`] produced.
+#[derive(Clone, Debug, PartialEq, Eq)]
+pub struct AuditOutcome {
+    pub record: AuditRecord,
+    /// Whether the record should be written. See [`Auditor::observe`].
+    pub emit: bool,
+}
+
+/// The dry-run auditor: phases 2–4 folded together over a live projection.
+///
+/// Read-only by construction — it borrows a [`Projection`] and owns only the
+/// state phases 2 and 4 thread explicitly ([`StickyState`], [`AecValidator`]).
+/// There is no field here through which a link could be created.
+#[derive(Clone, Debug)]
+pub struct Auditor {
+    validator: AecValidator,
+    sticky: StickyState,
+    seq: u64,
+    /// The body of the last record actually written, for change suppression.
+    last_emitted: Option<AuditBody>,
+}
+
+impl Auditor {
+    pub fn new(config: AuditConfig) -> Self {
+        Self {
+            validator: AecValidator::new(config.aec, config.aec_timeout),
+            sticky: StickyState::default(),
+            seq: 0,
+            last_emitted: None,
+        }
+    }
+
+    pub fn aec_state(&self) -> AecState {
+        self.validator.state()
+    }
+
+    pub fn sticky(&self) -> &StickyState {
+        &self.sticky
+    }
+
+    /// Fold one projection into the audit.
+    ///
+    /// **Called once per applied registry event — never on a coalesced batch.**
+    /// That is not a performance preference, it is the phase-4 integration
+    /// contract (`aec/mod.rs`, the `Validated` arm): revocation is detected by
+    /// observing the *empty gap* between a module unload and the next reload,
+    /// and module indices are reused verbatim (v3.4 §5.2 correction 3). Coalesce
+    /// across that gap and a fresh module silently inherits a dead module's
+    /// validated identity. [`sink`] is what upholds this, by running the
+    /// recompute inline on the observer thread rather than polling
+    /// [`RegistryObserverHandle::latest`](crate::host::observer::adapter::RegistryObserverHandle::latest),
+    /// which coalesces by nature.
+    ///
+    /// `emit` is true for every graph-triggered recompute, and for a
+    /// tick-triggered one only when the decision content changed. Ticks arrive
+    /// at a constant 4 Hz purely to drive the AEC deadline; emitting an
+    /// identical record four times a second would bury the graph events the
+    /// audit exists to show. `seq` still advances on suppressed records, so
+    /// nothing about the run is silently unaccounted for.
+    pub fn observe(
+        &mut self,
+        projection: &Projection,
+        kind: EventKind,
+        now: Millis,
+    ) -> AuditOutcome {
+        self.seq += 1;
+
+        // Phase 4 first: its verdict is an *input* to phase 2 via
+        // `ExclusionCtx::aec_module_id`, so observing the graph in the other
+        // order would evaluate taint against the previous recompute's identity.
+        self.validator
+            .observe(&projection.snapshot, projection.graph_ready, now);
+        let aec_state = self.validator.state();
+        let gate_reason = GateReason::from_state(aec_state);
+
+        let ctx = ExclusionCtx {
+            aec_module_id: self.validator.validated_module_id(),
+            pipewire_pulse_pid: projection.pipewire_pulse_pid,
+            // The audit creates nothing, so it owns nothing. Another host's
+            // capture sink is still caught — by the `pixelpass_capture_*` name
+            // prefix (v3.4 §6.2), which is what §5.1 row 7 exercises — so an
+            // empty set costs the matrix nothing.
+            pixelpass_owned: Default::default(),
+            graph_ready: projection.graph_ready,
+        };
+
+        let (decisions, sticky) = evaluate(&projection.snapshot, &ctx, &self.sticky);
+        self.sticky = sticky;
+
+        let body = build_body(
+            projection,
+            &decisions,
+            aec_state,
+            self.validator.validated_module_id(),
+            gate_reason,
+        );
+
+        let emit = kind == EventKind::Graph || self.last_emitted.as_ref() != Some(&body);
+        if emit {
+            self.last_emitted = Some(body.clone());
+        }
+
+        AuditOutcome {
+            record: AuditRecord {
+                seq: self.seq,
+                trigger: kind.code(),
+                at_ms: now,
+                body,
+            },
+            emit,
+        }
+    }
+}
+
+fn build_body(
+    projection: &Projection,
+    decisions: &Decisions,
+    aec_state: AecState,
+    aec_module_id: Option<u64>,
+    gate_reason: Option<GateReason>,
+) -> AuditBody {
+    let candidates: Vec<AuditRow> = decisions
+        .candidates
+        .values()
+        .map(|decision| {
+            // The engine's own reason wins when it has one: it names the
+            // mechanism that actually excluded *this* node, which is what the
+            // §5.1 rows assert. The gate reason applies only to candidates the
+            // engine would have passed — otherwise a shut gate would erase every
+            // reason code in the record and the matrix would stop constraining
+            // the engine at all.
+            let (eligible, reason, sticky) = match decision.eligibility {
+                Eligibility::NotEligible { reason, sticky } => (false, Some(reason.code()), sticky),
+                Eligibility::Eligible => match gate_reason {
+                    Some(gate) => (false, Some(gate.code()), false),
+                    None => (true, None, false),
+                },
+            };
+            AuditRow {
+                serial: decision.serial.0,
+                name: decision.name.clone(),
+                eligible,
+                reason,
+                sticky,
+            }
+        })
+        .collect();
+
+    let eligible_count = candidates.iter().filter(|row| row.eligible).count();
+
+    let taint: Vec<TaintRow> = decisions
+        .taint
+        .iter()
+        .map(|(&serial, entry)| TaintRow {
+            serial: serial.0,
+            name: node_name(projection, serial),
+            reason: entry.reason.code(),
+            sticky: entry.sticky,
+        })
+        .collect();
+
+    let ignored_ownership_tags: Vec<IgnoredTagRow> =
+        crate::host::taint::misplaced_ownership_tags(&projection.snapshot)
+            .into_iter()
+            .map(|node| IgnoredTagRow {
+                serial: node.serial.0,
+                name: node.name.clone(),
+                role: node.role.code(),
+            })
+            .collect();
+
+    AuditBody {
+        graph_ready: projection.graph_ready,
+        epoch: readiness_code(projection.readiness),
+        aec_state: aec_state_code(aec_state),
+        aec_module_id,
+        fan_out_permitted: gate_reason.is_none(),
+        gate_reason: gate_reason.map(GateReason::code),
+        excluded_count: candidates.len() - eligible_count,
+        eligible_count,
+        candidates,
+        taint,
+        ignored_ownership_tags,
+    }
+}
+
+fn node_name(projection: &Projection, serial: Serial) -> Option<String> {
+    projection
+        .snapshot
+        .node(serial)
+        .and_then(|node| node.name.clone())
+}

src/host/audit/run.rs

@@ -0,0 +1,170 @@
+//! Triggering the dry-run audit: environment parsing and the two entry points.
+//!
+//! The impl plan §5 specifies a **hidden trigger**, `PIXELPASS_AUDIO_AUDIT=1`.
+//! It is honoured in two places, which answer two different questions:
+//!
+//! - **Inside a real `pixelpass host` run** ([`spawn_if_enabled`]) — proves the
+//!   audit works in the code path phase 6 will actually mutate. This is the
+//!   plan-literal reading of the trigger.
+//! - **Standalone** ([`run_standalone`], behind the hidden `--audit-audio`
+//!   flag) — observer plus auditor and nothing else: no iroh endpoint, no
+//!   display-server detection, no capture pipeline, no ticket. This is what
+//!   drives the §5.1 matrix, because a row that fails should fail for a reason
+//!   about *audio*, not because a relay was unreachable.
+//!
+//! Both paths run the same [`AuditSink`] over the same observer, so neither is a
+//! simulation of the other.
+
+use std::fs::OpenOptions;
+use std::io::Write;
+
+use anyhow::{Context, Result, bail};
+
+use super::sink::AuditSink;
+use super::{AEC_VALIDATION_TIMEOUT_MILLIS, AuditConfig};
+use crate::common::signal;
+use crate::host::aec::{AecConfig, AecParseError, parse_aec_arg};
+use crate::host::observer::adapter::RegistryObserverHandle;
+
+/// The hidden trigger (impl plan §5). Exactly `1` enables the audit; anything
+/// else, including `true` or `yes`, does not.
+///
+/// Deliberately strict. This variable can only arrive by someone typing it, and
+/// a value that *looks* enabling but is not would produce a silent no-op — the
+/// single most annoying failure mode for a diagnostic tool. A mistyped value
+/// gets a warning (see [`enabled`]) rather than silence.
+pub const AUDIT_ENV: &str = "PIXELPASS_AUDIO_AUDIT";
+
+/// The AEC identity for the audit, in the `--aec` grammar (`off` or
+/// `pulse-module:<idx>`). Absent ⇒ `off`.
+pub const AUDIT_AEC_ENV: &str = "PIXELPASS_AUDIO_AUDIT_AEC";
+
+/// Redirect the JSON Lines stream to this file instead of stderr.
+pub const AUDIT_FILE_ENV: &str = "PIXELPASS_AUDIO_AUDIT_FILE";
+
+/// Whether the hidden trigger is set.
+pub fn enabled() -> bool {
+    match std::env::var(AUDIT_ENV) {
+        Ok(value) if value == "1" => true,
+        Ok(value) => {
+            tracing::warn!(
+                "{AUDIT_ENV}={value:?} is not `1`; the audio audit stays off. \
+                 Set {AUDIT_ENV}=1 to enable it."
+            );
+            false
+        }
+        Err(_) => false,
+    }
+}
+
+/// Build the audit configuration from the environment.
+///
+/// A malformed `PIXELPASS_AUDIO_AUDIT_AEC` is **fatal**, matching the phase-4
+/// rule that a bad `--aec` value must not silently become "no AEC": there is no
+/// fail-closed default index, so a wrong or dropped one would exclude the wrong
+/// node (or nothing at all) and the audit would confidently report a partition
+/// computed against an identity nobody asked for.
+pub fn config_from_env() -> Result<AuditConfig> {
+    let aec = match std::env::var(AUDIT_AEC_ENV) {
+        Ok(raw) => parse_aec_arg(&raw).map_err(|e| {
+            anyhow::anyhow!(
+                "{AUDIT_AEC_ENV}={raw:?} is not a valid AEC argument ({}). \
+                 Expected `off` or `pulse-module:<index>`, where the index is a bare decimal.",
+                describe(e)
+            )
+        })?,
+        Err(std::env::VarError::NotPresent) => AecConfig::Off,
+        Err(e) => bail!("{AUDIT_AEC_ENV} is not readable: {e}"),
+    };
+    Ok(AuditConfig {
+        aec,
+        aec_timeout: AEC_VALIDATION_TIMEOUT_MILLIS,
+    })
+}
+
+fn describe(error: AecParseError) -> &'static str {
+    match error {
+        AecParseError::Empty => "the value was empty",
+        AecParseError::UnknownForm => "not `off` and not `pulse-module:...`",
+        AecParseError::MissingIndex => "`pulse-module:` with no index after the colon",
+        AecParseError::InvalidIndex => {
+            "the index was not a bare decimal (no sign, whitespace, or non-digits) that fits in u64"
+        }
+    }
+}
+
+/// Where the JSON Lines go. Stderr unless `PIXELPASS_AUDIO_AUDIT_FILE` names a
+/// file, which is appended to rather than truncated — a matrix run that restarts
+/// the process mid-scenario should not lose the rows it already recorded.
+fn writer_from_env() -> Result<Box<dyn Write + Send>> {
+    match std::env::var(AUDIT_FILE_ENV) {
+        Ok(path) if !path.is_empty() => {
+            let file = OpenOptions::new()
+                .create(true)
+                .append(true)
+                .open(&path)
+                .with_context(|| format!("{AUDIT_FILE_ENV}={path:?} could not be opened"))?;
+            tracing::info!("audio audit: writing records to {path}");
+            Ok(Box::new(file))
+        }
+        _ => Ok(Box::new(std::io::stderr())),
+    }
+}
+
+/// Construct the sink and spawn the observer behind it.
+fn spawn_audit() -> Result<RegistryObserverHandle> {
+    let config = config_from_env()?;
+    let sink = AuditSink::new(config, writer_from_env()?);
+    tracing::info!(
+        aec = ?config.aec,
+        "audio audit: dry run active — decisions are logged, no links are created"
+    );
+    RegistryObserverHandle::spawn_with_sink(Some(Box::new(sink)))
+}
+
+/// Start the audit if the hidden trigger is set, for a `pixelpass host` run.
+///
+/// The returned handle must be held for the lifetime of the run: dropping it
+/// stops the observer thread and flushes the final O5 summary.
+///
+/// Returns `Err` only when the trigger *was* set and starting failed — a
+/// misconfigured audit is worth failing the run over, because the alternative is
+/// a host that silently is not being audited while its operator believes it is.
+pub fn spawn_if_enabled() -> Result<Option<RegistryObserverHandle>> {
+    if !enabled() {
+        return Ok(None);
+    }
+    spawn_audit().map(Some)
+}
+
+/// The standalone audit: run the observer and the auditor, and nothing else,
+/// until ctrl-c.
+///
+/// Does not consult [`AUDIT_ENV`] — reaching this function required passing the
+/// hidden `--audit-audio` flag, which is already an explicit request. The
+/// environment still supplies the AEC identity and the output file.
+pub async fn run_standalone() -> Result<()> {
+    let cancel = signal::install_ctrl_c();
+    let handle = spawn_audit()?;
+
+    eprintln!(
+        "pixelpass audio audit (dry run): observing the live PipeWire graph.\n\
+         No links are created and no routing changes. Ctrl-C to stop."
+    );
+
+    // SIGTERM as well as ctrl-c, because this mode is driven by scripts as much
+    // as by hand — `timeout`, a matrix harness, and systemd all send SIGTERM,
+    // and the default disposition would kill the process before the sink's
+    // `Drop` writes the final O5 summary. Losing that summary is losing the
+    // whole §5.2 measurement for that run.
+    let mut sigterm = signal::terminate_stream()?;
+    tokio::select! {
+        _ = cancel.cancelled() => {}
+        _ = sigterm.recv() => tracing::info!("SIGTERM received, shutting down"),
+    }
+    // Explicit rather than incidental: this drop stops the PipeWire thread,
+    // which drops the sink, which writes the final metrics line. Letting it fall
+    // out of scope would do the same thing, but the ordering is the point.
+    drop(handle);
+    Ok(())
+}

src/host/audit/sink.rs

@@ -0,0 +1,184 @@
+//! The audit's I/O edge: timing, JSON Lines emission, O5 accounting.
+//!
+//! Everything impure about phase 5 lives here, and it is deliberately thin —
+//! read the clock, call [`Auditor::observe`], write a line, fold a
+//! [`metrics::Sample`]. The decisions are all upstream in the pure core, which
+//! is why the matrix can be argued about in unit tests rather than only in front
+//! of a live daemon.
+//!
+//! ## Why this runs on the observer thread
+//!
+//! [`AuditSink`] is a [`ProjectionSink`], invoked inline from the PipeWire
+//! observer thread once per applied registry event. The obvious alternative —
+//! a consumer task polling
+//! [`RegistryObserverHandle::latest`](super::super::observer::adapter::RegistryObserverHandle::latest)
+//! — was rejected: polling **coalesces**, and phase 4's revocation logic
+//! detects a module unload by observing the *empty gap* before the next module
+//! appears. Module indices are reused verbatim across an unload/reload (v3.4
+//! §5.2 correction 3), so a poller that misses the gap silently aliases a fresh
+//! module onto a dead module's validated identity. Running inline is what makes
+//! "one `observe` per graph event, no coalescing" — the contract phase 4
+//! documents as owed — actually true.
+//!
+//! The cost of that choice is that recompute and logging happen on the thread
+//! servicing PipeWire, which is precisely the risk O5 asks about. That is not an
+//! accident: this arrangement puts the cost exactly where the measurement can
+//! see it. See [`metrics`].
+//!
+//! ## Output contract
+//!
+//! One JSON object per line, to **stderr** by default, each tagged with a `kind`
+//! discriminator (`"audit"` or `"metrics"`). Never stdout: peerspeak parses
+//! pixelpass's stdout event stream, and the impl plan §5 is explicit that
+//! unstructured output must not go there. `PIXELPASS_AUDIO_AUDIT_FILE`
+//! redirects the records to a file instead, which is how the §5.1 matrix is
+//! driven — it separates the audit stream from interleaved `tracing` output
+//! without needing either side to change format.
+
+use std::io::Write;
+use std::time::Instant;
+
+use serde::Serialize;
+
+use super::metrics::{self, Metrics, Summary};
+use super::{AuditConfig, AuditRecord, Auditor};
+use crate::host::observer::adapter::ProjectionSink;
+use crate::host::observer::{EventKind, Millis, Projection};
+
+/// Emit a rolling metrics line every this many ticks. Ticks are 250 ms, so this
+/// is every 10 s — often enough that a run killed abruptly still leaves a
+/// usable O5 record, rare enough that it does not crowd out the audit records.
+const SUMMARY_INTERVAL_TICKS: u64 = 40;
+
+/// The live audit: pure auditor + clock + writer.
+pub struct AuditSink {
+    auditor: Auditor,
+    metrics: Metrics,
+    writer: Box<dyn Write + Send>,
+    /// Set once the first sample has completed, so the first event is not
+    /// counted as having queued behind a predecessor that does not exist.
+    last_completion_us: Option<u64>,
+    ticks_since_summary: u64,
+    /// Wall-clock origin for the microsecond timings. Only used for durations,
+    /// never for the AEC deadline — that runs on the observer's own clock,
+    /// handed in as `now_us`, so the validator and the readiness epoch cannot
+    /// disagree about what time it is.
+    epoch: Instant,
+}
+
+impl AuditSink {
+    pub fn new(config: AuditConfig, writer: Box<dyn Write + Send>) -> Self {
+        Self {
+            auditor: Auditor::new(config),
+            metrics: Metrics::default(),
+            writer,
+            last_completion_us: None,
+            ticks_since_summary: 0,
+            epoch: Instant::now(),
+        }
+    }
+
+    fn elapsed_us(&self) -> u64 {
+        u64::try_from(self.epoch.elapsed().as_micros()).unwrap_or(u64::MAX)
+    }
+
+    /// Write one line. Failures are logged once per occurrence and otherwise
+    /// ignored: a broken stderr must not take down the observer thread, and the
+    /// audit is diagnostic — losing a line is a worse audit, not a worse share.
+    fn write_line<T: Serialize>(&mut self, line: &T) {
+        match serde_json::to_string(line) {
+            Ok(json) => {
+                if let Err(e) = writeln!(self.writer, "{json}") {
+                    tracing::warn!("audit: failed to write record: {e}");
+                }
+            }
+            Err(e) => tracing::warn!("audit: failed to serialise record: {e}"),
+        }
+    }
+
+    fn write_summary(&mut self, at_ms: Millis) {
+        let summary = self.metrics.summary();
+        self.write_line(&MetricsLine {
+            kind: "metrics",
+            at_ms,
+            summary: &summary,
+        });
+        let _ = self.writer.flush();
+    }
+}
+
+impl ProjectionSink for AuditSink {
+    fn on_projection(&mut self, projection: &Projection, kind: EventKind, now_us: u64) {
+        let at_us = self.elapsed_us();
+        let gap_us = self
+            .last_completion_us
+            .map(|previous| at_us.saturating_sub(previous))
+            .unwrap_or(0);
+
+        let recompute_start = self.elapsed_us();
+        let outcome = self.auditor.observe(projection, kind, now_us / 1_000);
+        let recompute_us = self.elapsed_us().saturating_sub(recompute_start);
+
+        let emit_us = if outcome.emit {
+            let emit_start = self.elapsed_us();
+            self.write_line(&AuditLine {
+                kind: "audit",
+                recompute_us,
+                record: &outcome.record,
+            });
+            // Flushed per record so a run ended with SIGKILL (or a matrix row
+            // that reads the file while the process is still up) still shows
+            // every decision made before that instant. The cost is measured, not
+            // assumed — it is inside `emit_us`.
+            let _ = self.writer.flush();
+            self.elapsed_us().saturating_sub(emit_start).max(1)
+        } else {
+            0
+        };
+
+        self.metrics.record(metrics::Sample {
+            at_us,
+            gap_us,
+            recompute_us,
+            emit_us,
+            kind,
+        });
+        self.last_completion_us = Some(self.elapsed_us());
+
+        if kind == EventKind::Tick {
+            self.ticks_since_summary += 1;
+            if self.ticks_since_summary >= SUMMARY_INTERVAL_TICKS {
+                self.ticks_since_summary = 0;
+                self.write_summary(now_us / 1_000);
+            }
+        }
+    }
+}
+
+impl Drop for AuditSink {
+    /// The final O5 record. The observer thread drops its sink when the main
+    /// loop quits, so an ordinary ctrl-c leaves a complete summary behind
+    /// without the runner having to ask for one.
+    fn drop(&mut self) {
+        let at_ms = self.elapsed_us() / 1_000;
+        self.write_summary(at_ms);
+    }
+}
+
+#[derive(Serialize)]
+struct AuditLine<'a> {
+    kind: &'static str,
+    /// This record's own recompute cost, so a surprising row can be correlated
+    /// with a cost spike without cross-referencing the periodic summary.
+    recompute_us: u64,
+    #[serde(flatten)]
+    record: &'a AuditRecord,
+}
+
+#[derive(Serialize)]
+struct MetricsLine<'a> {
+    kind: &'static str,
+    at_ms: Millis,
+    #[serde(flatten)]
+    summary: &'a Summary,
+}

src/host/audit/tests.rs

@@ -0,0 +1,960 @@
+//! Pure tests for the phase-5 auditor and its O5 metrics.
+//!
+//! Two things are being tested here and they are worth keeping distinct:
+//!
+//! - **Audit-layer behaviour** — the fan-out gate, record suppression, sequence
+//!   accounting, epoch reporting, and above all that every record carries the
+//!   *complete* candidate universe (§5.1). These are properties nothing else
+//!   tests, because nothing else exists at this layer.
+//! - **A few §5.1 matrix shapes in fixture form** — row 1 (owner-bridge
+//!   forwarder), row 3 (two modules, one tainted), row 12 (AEC lifecycle). These
+//!   are *not* re-litigating phase 2, whose 57 tests already own those verdicts.
+//!   They exist so that a plumbing mistake between the engine and the record —
+//!   a dropped reason code, an inverted partition — fails here, at compile-time
+//!   speed, rather than only in front of a live daemon.
+//!
+//! The live half of the gate cannot live in this file by definition: a fixture
+//! tests my model against my own assumptions, and §5's whole argument is that
+//! only a live run tests my model against PipeWire. See the matrix runs recorded
+//! in the phase-5 results file.
+
+use super::metrics::{BUCKET_LABELS, Metrics, QUEUE_THRESHOLD_US, Sample};
+use super::*;
+use crate::host::aec::AecConfig;
+use crate::host::observer::{EventKind, Readiness};
+use crate::host::taint::PEERSPEAK_OWNED_NODE_PREFIX;
+use crate::host::taint::fixture::{self, Graph, NodeRef};
+use crate::host::taint::snapshot::{GraphSnapshot, MediaRole};
+
+/// The `node.name` a [`Graph::peerspeak_node`] fixture produces. Built from
+/// the same constant the engine matches on, so these audit rows report the
+/// name shape a live peerspeak node actually has (v3.5 §5.1, carrier 2).
+fn owned_name(role: &str, pid: u32) -> String {
+    format!("{PEERSPEAK_OWNED_NODE_PREFIX}{role}_{pid}")
+}
+
+const AEC_MODULE: u64 = 7;
+const TIMEOUT: Millis = 5_000;
+
+fn ready(snapshot: GraphSnapshot) -> Projection {
+    Projection {
+        snapshot,
+        pipewire_pulse_pid: Some(fixture::PULSE_PID),
+        graph_ready: true,
+        readiness: Readiness::Complete,
+    }
+}
+
+fn not_ready(snapshot: GraphSnapshot, readiness: Readiness) -> Projection {
+    Projection {
+        snapshot,
+        pipewire_pulse_pid: Some(fixture::PULSE_PID),
+        graph_ready: false,
+        readiness,
+    }
+}
+
+fn auditor_off() -> Auditor {
+    Auditor::new(AuditConfig {
+        aec: AecConfig::Off,
+        aec_timeout: TIMEOUT,
+    })
+}
+
+fn auditor_aec(index: u64) -> Auditor {
+    Auditor::new(AuditConfig {
+        aec: AecConfig::PulseModule(index),
+        aec_timeout: TIMEOUT,
+    })
+}
+
+/// One graph-triggered recompute at `now`.
+fn observe(auditor: &mut Auditor, projection: &Projection, now: Millis) -> AuditOutcome {
+    auditor.observe(projection, EventKind::Graph, now)
+}
+
+/// Candidate names split into (eligible, excluded-with-reason), which is how the
+/// §5.1 rows are phrased. Names rather than serials so a failure reads as the
+/// scenario rather than as an integer.
+fn partition(body: &AuditBody) -> (Vec<&str>, Vec<(&str, &str)>) {
+    let eligible = body
+        .candidates
+        .iter()
+        .filter(|row| row.eligible)
+        .map(|row| row.name.as_deref().unwrap_or("<unnamed>"))
+        .collect();
+    let excluded = body
+        .candidates
+        .iter()
+        .filter(|row| !row.eligible)
+        .map(|row| {
+            (
+                row.name.as_deref().unwrap_or("<unnamed>"),
+                row.reason.unwrap_or("<none>"),
+            )
+        })
+        .collect();
+    (eligible, excluded)
+}
+
+// ── the §5.1 structural requirement ───────────────────────────────────────
+
+/// The record must contain **every** `Stream/Output/Audio` node, not only the
+/// interesting ones. This is the property the whole exact-partition requirement
+/// rests on: if the record could omit a candidate, then asserting a complete
+/// partition over the record would still not constrain the graph.
+#[test]
+fn the_record_carries_the_complete_candidate_universe() {
+    let mut graph = Graph::new();
+    graph.app_node("music", MediaRole::StreamOutput, 100);
+    graph.app_node("game", MediaRole::StreamOutput, 101);
+    graph.app_node("recorder", MediaRole::StreamInput, 102);
+    graph.device_node("speakers", MediaRole::Sink);
+    let projection = ready(graph.build());
+
+    let outcome = observe(&mut auditor_off(), &projection, 0);
+    let (eligible, excluded) = partition(&outcome.record.body);
+
+    // Both playback streams, neither the capture stream nor the sink. Ordered by
+    // serial (creation order), which is what makes the partition assertions in
+    // every other row stable rather than dependent on a hash iteration.
+    assert_eq!(eligible, vec!["music", "game"]);
+    assert!(excluded.is_empty(), "unexpected exclusions: {excluded:?}");
+    assert_eq!(outcome.record.body.candidates.len(), 2);
+    assert_eq!(outcome.record.body.eligible_count, 2);
+    assert_eq!(outcome.record.body.excluded_count, 0);
+}
+
+/// **R10-1's diagnostic reaches the record.** The engine deliberately ignores
+/// an ownership carrier on a non-producer, which means the fix removes an
+/// exclusion — so the only way an operator learns a tag was seen and dropped is
+/// this field. A matrix row that silently grew an impostor would otherwise read
+/// as a clean pass.
+#[test]
+fn an_ignored_ownership_tag_is_reported_without_excluding_anything() {
+    let mut graph = Graph::new();
+    graph.app_node("music", MediaRole::StreamOutput, 100);
+    let impostor = graph.peerspeak_tagged_node("rogue", MediaRole::StreamInput, 4_242);
+    let projection = ready(graph.build());
+
+    let body = observe(&mut auditor_off(), &projection, 0).record.body;
+
+    // The bystander is untouched — the point of the fix.
+    let (eligible, excluded) = partition(&body);
+    assert_eq!(eligible, vec!["music"]);
+    assert!(excluded.is_empty(), "unexpected exclusions: {excluded:?}");
+    assert!(body.taint.is_empty(), "unexpected taint: {:?}", body.taint);
+
+    // ...but the tag is not silent, and the row names the role it appeared on.
+    assert_eq!(body.ignored_ownership_tags.len(), 1);
+    let row = &body.ignored_ownership_tags[0];
+    assert_eq!(row.serial, impostor.serial.0);
+    assert_eq!(row.role, "stream-input");
+    assert_eq!(
+        row.name.as_deref(),
+        Some(owned_name("rogue", 4_242).as_str())
+    );
+}
+
+/// The common path stays quiet: a correctly tagged peerspeak producer is
+/// honoured as a taint root and is *not* reported as a misplaced tag. Without
+/// this, a diagnostic that fired on every normal run would be worthless.
+#[test]
+fn a_correctly_tagged_producer_is_not_reported_as_misplaced() {
+    let mut graph = Graph::new();
+    let sink = graph.device_node("speakers", MediaRole::Sink);
+    let call = graph.peerspeak_node("call", 200);
+    graph.link(call, sink);
+    let projection = ready(graph.build());
+
+    let body = observe(&mut auditor_off(), &projection, 0).record.body;
+
+    assert_eq!(body.excluded_count, 1);
+    assert!(
+        body.ignored_ownership_tags.is_empty(),
+        "honoured tag reported as misplaced: {:?}",
+        body.ignored_ownership_tags
+    );
+}
+
+/// The fail-closed default asserted at the boundary (impl plan §4, phase 2's
+/// "one addition"): nothing in, nothing eligible — and, just as importantly, no
+/// panic and no invented row.
+#[test]
+fn an_empty_graph_yields_an_empty_partition() {
+    let projection = ready(Graph::new().build());
+    let outcome = observe(&mut auditor_off(), &projection, 0);
+
+    assert!(outcome.record.body.candidates.is_empty());
+    assert!(outcome.record.body.taint.is_empty());
+    assert_eq!(outcome.record.body.eligible_count, 0);
+    assert_eq!(outcome.record.body.excluded_count, 0);
+    assert!(outcome.record.body.fan_out_permitted);
+}
+
+/// `eligible_count + excluded_count` is the candidate count, always. A partition
+/// that does not partition would let a row's two assertions both pass while the
+/// record described no coherent state.
+#[test]
+fn the_counts_always_partition_the_candidates() {
+    let mut graph = Graph::new();
+    graph.app_node("music", MediaRole::StreamOutput, 100);
+    graph.peerspeak_node("peerspeak-playback", 200);
+    let projection = ready(graph.build());
+
+    let body = observe(&mut auditor_off(), &projection, 0).record.body;
+    assert_eq!(
+        body.eligible_count + body.excluded_count,
+        body.candidates.len()
+    );
+    assert_eq!(body.eligible().len(), body.eligible_count);
+    assert_eq!(body.excluded().len(), body.excluded_count);
+}
+
+/// Every excluded row names a reason and every eligible row does not. The
+/// §5.1 rows assert "excluded, with reason code" — a `None` reason on an
+/// excluded row would make that assertion unwritable.
+#[test]
+fn reason_presence_is_exactly_the_exclusion() {
+    let mut graph = Graph::new();
+    graph.app_node("music", MediaRole::StreamOutput, 100);
+    graph.peerspeak_node("peerspeak-playback", 200);
+    let projection = ready(graph.build());
+
+    for row in observe(&mut auditor_off(), &projection, 0)
+        .record
+        .body
+        .candidates
+    {
+        assert_eq!(
+            row.eligible,
+            row.reason.is_none(),
+            "row {row:?} has eligibility and reason out of step"
+        );
+    }
+}
+
+// ── readiness ─────────────────────────────────────────────────────────────
+
+/// No decision made from a partial graph is a decision. Note this is asserted on
+/// the *eligible* half too: an implementation that reported nothing at all while
+/// not ready would also be wrong, because the audit must still show what it can
+/// see.
+#[test]
+fn a_not_ready_graph_excludes_every_candidate() {
+    let mut graph = Graph::new();
+    graph.app_node("music", MediaRole::StreamOutput, 100);
+    graph.app_node("game", MediaRole::StreamOutput, 101);
+    let projection = not_ready(graph.build(), Readiness::Waiting);
+
+    let body = observe(&mut auditor_off(), &projection, 0).record.body;
+    let (eligible, excluded) = partition(&body);
+
+    assert!(eligible.is_empty());
+    assert_eq!(
+        excluded,
+        vec![("music", "graph-not-ready"), ("game", "graph-not-ready"),]
+    );
+    assert!(!body.graph_ready);
+}
+
+/// The three ways `graph_ready` can be false are distinguishable in the record.
+/// Collapsing them would make a timed-out observer — a fail-closed *fault* —
+/// indistinguishable from an enumeration that is merely still running.
+#[test]
+fn the_epoch_distinguishes_the_ways_a_graph_can_be_unready() {
+    let mut graph = Graph::new();
+    graph.app_node("music", MediaRole::StreamOutput, 100);
+    let snapshot = graph.build();
+
+    for (readiness, expected) in [
+        (Readiness::Waiting, "waiting"),
+        (Readiness::TimedOut, "timed-out"),
+        // A completed epoch momentarily blocked on a current obligation: the
+        // interesting one, because `graph_ready` alone makes it look like a
+        // brand-new observer.
+        (Readiness::Complete, "complete"),
+    ] {
+        let projection = not_ready(snapshot.clone(), readiness);
+        let body = observe(&mut auditor_off(), &projection, 0).record.body;
+        assert_eq!(body.epoch, expected);
+        assert!(!body.graph_ready);
+    }
+
+    let body = observe(&mut auditor_off(), &ready(snapshot), 0).record.body;
+    assert_eq!(body.epoch, "complete");
+    assert!(body.graph_ready);
+}
+
+// ── the fan-out gate (phase 4 → audit) ────────────────────────────────────
+
+/// `--aec=off` leaves the gate open: `NotConfigured` is "there is no echo
+/// canceller", not "we failed to find one".
+#[test]
+fn aec_off_leaves_the_gate_open() {
+    let mut graph = Graph::new();
+    graph.app_node("music", MediaRole::StreamOutput, 100);
+    let projection = ready(graph.build());
+
+    let body = observe(&mut auditor_off(), &projection, 0).record.body;
+    assert_eq!(body.aec_state, "not-configured");
+    assert!(body.fan_out_permitted);
+    assert_eq!(body.gate_reason, None);
+    assert_eq!(body.aec_module_id, None);
+    assert_eq!(partition(&body).0, vec!["music"]);
+}
+
+/// While the configured identity has not been seen, nothing may fan out —
+/// silence over echo — and the record says why in a code, not in prose.
+#[test]
+fn a_validating_gate_excludes_every_engine_eligible_candidate() {
+    let mut graph = Graph::new();
+    graph.app_node("music", MediaRole::StreamOutput, 100);
+    graph.app_node("game", MediaRole::StreamOutput, 101);
+    let projection = ready(graph.build());
+
+    let body = observe(&mut auditor_aec(AEC_MODULE), &projection, 0)
+        .record
+        .body;
+    let (eligible, excluded) = partition(&body);
+
+    assert_eq!(body.aec_state, "validating");
+    assert!(!body.fan_out_permitted);
+    assert_eq!(body.gate_reason, Some("aec-validating"));
+    assert!(eligible.is_empty());
+    assert_eq!(
+        excluded,
+        vec![("music", "aec-validating"), ("game", "aec-validating")]
+    );
+}
+
+/// A shut gate must not erase the engine's own reason codes. If it did, every
+/// §5.1 row run under a shut gate would report one uniform code and the matrix
+/// would stop constraining the taint engine at all — the record would say
+/// "nothing may fan out" while hiding *which* nodes were tainted and how.
+#[test]
+fn a_shut_gate_preserves_the_engines_own_reasons() {
+    let mut graph = Graph::new();
+    graph.app_node("music", MediaRole::StreamOutput, 100);
+    graph.peerspeak_node("peerspeak-playback", 200);
+    let projection = ready(graph.build());
+
+    let body = observe(&mut auditor_aec(AEC_MODULE), &projection, 0)
+        .record
+        .body;
+    let (_, excluded) = partition(&body);
+
+    let playback = owned_name("peerspeak-playback", 200);
+    assert!(!body.fan_out_permitted);
+    assert_eq!(
+        excluded,
+        vec![
+            ("music", "aec-validating"),
+            // Tagged, so it keeps the reason that actually applies to it.
+            (playback.as_str(), "peerspeak-owned"),
+        ]
+    );
+}
+
+/// The deadline is armed on the first ready graph, so a slow enumeration reads
+/// as "unknown", not "absent" (the phase-4 user design call). Past it with the
+/// identity never seen, the gate latches shut.
+#[test]
+fn the_gate_fails_closed_after_the_deadline() {
+    let mut graph = Graph::new();
+    graph.app_node("music", MediaRole::StreamOutput, 100);
+    let snapshot = graph.build();
+    let mut auditor = auditor_aec(AEC_MODULE);
+
+    // Still enumerating well past the timeout: not a failure, because absence
+    // has not been established.
+    let waiting = not_ready(snapshot.clone(), Readiness::Waiting);
+    let body = observe(&mut auditor, &waiting, TIMEOUT * 3).record.body;
+    assert_eq!(body.aec_state, "validating");
+
+    // Ready arms the deadline; the clock has to advance past it from here.
+    let projection = ready(snapshot);
+    let body = observe(&mut auditor, &projection, TIMEOUT * 3).record.body;
+    assert_eq!(body.aec_state, "validating");
+
+    let body = observe(&mut auditor, &projection, TIMEOUT * 6 + 1)
+        .record
+        .body;
+    assert_eq!(body.aec_state, "failed");
+    assert_eq!(body.gate_reason, Some("aec-failed"));
+    assert_eq!(partition(&body).1, vec![("music", "aec-failed")]);
+}
+
+// ── §5.1 row 12: the AEC lifecycle ────────────────────────────────────────
+
+/// Build the four nodes `module-echo-cancel` creates, all bearing one index:
+/// two `Stream/*` legs plus the virtual sink/source pair (v3.4 §5.2). The
+/// playback leg is the hazard — a `Stream/Output/Audio` wired to the speakers.
+fn aec_nodes(graph: &mut Graph, index: u64) -> Vec<NodeRef> {
+    vec![
+        graph.module_node("echo-cancel-playback", MediaRole::StreamOutput, index),
+        graph.module_node("echo-cancel-capture", MediaRole::StreamInput, index),
+        graph.module_node("echo-cancel-sink", MediaRole::Sink, index),
+        graph.module_node("echo-cancel-source", MediaRole::Source, index),
+    ]
+}
+
+/// §5.1 row 12: AEC loaded → validated, its playback leg excluded by identity
+/// while everything else stays eligible → unloaded → `Revoked`, gate shut.
+///
+/// The eligible half is the load-bearing assertion in the first phase: an
+/// implementation that excluded the whole graph the moment an AEC appeared would
+/// satisfy "the four nodes are excluded" and still be wrong.
+#[test]
+fn row_12_aec_loaded_then_unloaded_validates_then_revokes() {
+    let mut graph = Graph::new();
+    graph.app_node("music", MediaRole::StreamOutput, 100);
+    let aec = aec_nodes(&mut graph, AEC_MODULE);
+    let mut auditor = auditor_aec(AEC_MODULE);
+
+    let loaded = ready(graph.build());
+    let body = observe(&mut auditor, &loaded, 0).record.body;
+    let (eligible, excluded) = partition(&body);
+
+    assert_eq!(body.aec_state, "validated");
+    assert!(body.fan_out_permitted);
+    assert_eq!(body.aec_module_id, Some(AEC_MODULE));
+    assert_eq!(eligible, vec!["music"]);
+    assert_eq!(excluded, vec![("echo-cancel-playback", "aec-identity")]);
+
+    // Every node bearing the index goes away: a real unload.
+    let unloaded = ready(graph.build_without(&aec));
+    let body = observe(&mut auditor, &unloaded, 1).record.body;
+    let (eligible, excluded) = partition(&body);
+
+    assert_eq!(body.aec_state, "revoked");
+    assert!(!body.fan_out_permitted);
+    assert_eq!(body.gate_reason, Some("aec-revoked"));
+    assert_eq!(body.aec_module_id, None);
+    assert!(eligible.is_empty());
+    assert_eq!(excluded, vec![("music", "aec-revoked")]);
+}
+
+/// One leg corking is not a revocation (v3.4 §5.3). Getting this wrong turns an
+/// ordinary cork into a share-wide audio stop, so the audit must report the
+/// identity as still live.
+#[test]
+fn row_12_partial_leg_loss_does_not_revoke() {
+    let mut graph = Graph::new();
+    graph.app_node("music", MediaRole::StreamOutput, 100);
+    let aec = aec_nodes(&mut graph, AEC_MODULE);
+    let mut auditor = auditor_aec(AEC_MODULE);
+
+    let body = observe(&mut auditor, &ready(graph.build()), 0).record.body;
+    assert_eq!(body.aec_state, "validated");
+
+    // The capture leg alone disappears; three nodes still bear the index.
+    let partial = ready(graph.build_without(&aec[1..2]));
+    let body = observe(&mut auditor, &partial, 1).record.body;
+
+    assert_eq!(body.aec_state, "validated");
+    assert!(body.fan_out_permitted);
+    assert_eq!(partition(&body).0, vec!["music"]);
+}
+
+/// Revocation is sticky terminal: module indices are reused verbatim across an
+/// unload/reload (v3.4 §5.2 correction 3), so a reappearing index must not
+/// resurrect the epoch and alias onto an unrelated module. A genuine reload gets
+/// a fresh validator, never this one.
+#[test]
+fn row_12_a_reused_index_does_not_resurrect_a_revoked_epoch() {
+    let mut graph = Graph::new();
+    graph.app_node("music", MediaRole::StreamOutput, 100);
+    let aec = aec_nodes(&mut graph, AEC_MODULE);
+    let mut auditor = auditor_aec(AEC_MODULE);
+
+    observe(&mut auditor, &ready(graph.build()), 0);
+    let unloaded = graph.build_without(&aec);
+    let body = observe(&mut auditor, &ready(unloaded), 1).record.body;
+    assert_eq!(body.aec_state, "revoked");
+
+    // A second module comes back with the same index — different objects,
+    // identical number.
+    let mut reloaded = Graph::new();
+    reloaded.app_node("music", MediaRole::StreamOutput, 100);
+    aec_nodes(&mut reloaded, AEC_MODULE);
+    let body = observe(&mut auditor, &ready(reloaded.build()), 2)
+        .record
+        .body;
+
+    assert_eq!(body.aec_state, "revoked");
+    assert!(!body.fan_out_permitted);
+    assert_eq!(body.gate_reason, Some("aec-revoked"));
+}
+
+// ── §5.1 rows in fixture form (plumbing, not phase-2 verdicts) ────────────
+
+/// §5.1 row 1: a `module-null-sink` + `module-loopback` forwarder. The output
+/// leg is excluded across the **owner bridge** — naming the mechanism, not a
+/// link walk — while an identically-shaped forwarder with no tainted input stays
+/// eligible. The second half is what an exclude-everything build fails.
+#[test]
+fn row_1_owner_bridge_forwarder_with_an_untainted_control() {
+    let mut graph = Graph::new();
+    // Tainted root: peerspeak's own call playback, feeding a sink the forwarder
+    // reads back out.
+    let call = graph.peerspeak_node("peerspeak-call", 200);
+    let sink = graph.module_node("tainted-null-sink", MediaRole::Sink, 30);
+    graph.link(call, sink);
+    let capture = graph.module_node("tainted-loopback-capture", MediaRole::StreamInput, 30);
+    let playback = graph.module_node("tainted-loopback-playback", MediaRole::StreamOutput, 30);
+    graph.link(sink, capture);
+    let _ = playback;
+
+    // Control: the same shape, fed by nothing tainted.
+    let clean_sink = graph.module_node("clean-null-sink", MediaRole::Sink, 31);
+    let clean_capture = graph.module_node("clean-loopback-capture", MediaRole::StreamInput, 31);
+    let clean_playback = graph.module_node("clean-loopback-playback", MediaRole::StreamOutput, 31);
+    graph.link(clean_sink, clean_capture);
+    let _ = clean_playback;
+
+    let body = observe(&mut auditor_off(), &ready(graph.build()), 0)
+        .record
+        .body;
+    let (eligible, excluded) = partition(&body);
+
+    let call_name = owned_name("peerspeak-call", 200);
+    assert_eq!(eligible, vec!["clean-loopback-playback"]);
+    assert_eq!(
+        excluded,
+        vec![
+            (call_name.as_str(), "peerspeak-owned"),
+            ("tainted-loopback-playback", "tainted-owner-bridge"),
+        ]
+    );
+}
+
+/// §5.1 row 3: two Pulse modules, one tainted input. **The other module's output
+/// must be eligible** — this is the row that makes a wrong pipewire-pulse-PID
+/// fusion observable, because fusing all Pulse-created nodes into one owner
+/// would drag the innocent module's output leg down with the tainted one.
+#[test]
+fn row_3_one_tainted_module_does_not_taint_the_other() {
+    let mut graph = Graph::new();
+    let call = graph.peerspeak_node("peerspeak-call", 200);
+    let sink = graph.module_node("null-sink-a", MediaRole::Sink, 40);
+    graph.link(call, sink);
+    let capture_a = graph.module_node("module-a-capture", MediaRole::StreamInput, 40);
+    let playback_a = graph.module_node("module-a-playback", MediaRole::StreamOutput, 40);
+    graph.link(sink, capture_a);
+    let _ = playback_a;
+
+    // A second, entirely independent module reading an untainted source.
+    let mic = graph.device_node("microphone", MediaRole::Source);
+    let capture_b = graph.module_node("module-b-capture", MediaRole::StreamInput, 41);
+    let playback_b = graph.module_node("module-b-playback", MediaRole::StreamOutput, 41);
+    graph.link(mic, capture_b);
+    let _ = playback_b;
+
+    let body = observe(&mut auditor_off(), &ready(graph.build()), 0)
+        .record
+        .body;
+    let (eligible, excluded) = partition(&body);
+
+    let call_name = owned_name("peerspeak-call", 200);
+    assert_eq!(eligible, vec!["module-b-playback"]);
+    assert_eq!(
+        excluded,
+        vec![
+            (call_name.as_str(), "peerspeak-owned"),
+            ("module-a-playback", "tainted-owner-bridge"),
+        ]
+    );
+}
+
+/// §5.1 row 7 (cycle prevention, v3.4 §6.2): a forwarder reading *another*
+/// pixelpass host's capture sink must be excluded by its **named output
+/// serial**, or two hosts sharing to each other build an audio cycle.
+#[test]
+fn row_7_a_forwarder_reading_another_hosts_capture_sink_is_excluded() {
+    let mut graph = Graph::new();
+    // The other host's own client, in *this* graph — a node pointing at a client
+    // that does not exist would exercise the unresolved-owner path instead of the
+    // capture-sink-name path this row is about.
+    let other_client = graph.client(Some(fixture::PULSE_PID));
+    let other_sink = graph.node(
+        "pixelpass_capture_deadbeef",
+        MediaRole::Sink,
+        fixture::app(other_client, 300),
+    );
+    let capture = graph.module_node("cycle-loopback-capture", MediaRole::StreamInput, 50);
+    let playback = graph.module_node("cycle-loopback-playback", MediaRole::StreamOutput, 50);
+    graph.link(other_sink, capture);
+    let _ = playback;
+    graph.app_node("music", MediaRole::StreamOutput, 100);
+
+    let body = observe(&mut auditor_off(), &ready(graph.build()), 0)
+        .record
+        .body;
+    let (eligible, excluded) = partition(&body);
+
+    assert_eq!(eligible, vec!["music"]);
+    assert_eq!(
+        excluded,
+        vec![("cycle-loopback-playback", "tainted-owner-bridge")]
+    );
+    // The sink itself is tainted, by the mechanism that names it.
+    let sink_taint = body
+        .taint
+        .iter()
+        .find(|row| row.name.as_deref() == Some("pixelpass_capture_deadbeef"))
+        .expect("the other host's capture sink must be tainted");
+    assert_eq!(sink_taint.reason, "pixelpass-owned");
+}
+
+/// Sticky taint (§5.1 row 10) is reported as sticky, not silently folded into
+/// an ordinary exclusion. The flag is how the audit distinguishes "this is
+/// tainted right now" from "this was tainted and its owner has not fully torn
+/// down" — two different things to be surprised by.
+#[test]
+fn sticky_exclusions_are_flagged_as_sticky() {
+    let mut graph = Graph::new();
+    let call = graph.peerspeak_node("peerspeak-call", 200);
+    let sink = graph.module_node("null-sink", MediaRole::Sink, 60);
+    graph.link(call, sink);
+    let capture = graph.module_node("loopback-capture", MediaRole::StreamInput, 60);
+    graph.module_node("loopback-playback", MediaRole::StreamOutput, 60);
+    graph.link(sink, capture);
+
+    let mut auditor = auditor_off();
+    let body = observe(&mut auditor, &ready(graph.build()), 0).record.body;
+    let playback = body
+        .row_named("loopback-playback")
+        .expect("the output leg must be a candidate");
+    assert!(!playback.eligible);
+    assert!(!playback.sticky, "first sight is not sticky");
+
+    // The tainted input leg goes away; the output leg lives on.
+    let body = observe(&mut auditor, &ready(graph.build_without(&[capture])), 1)
+        .record
+        .body;
+    let playback = body
+        .row_named("loopback-playback")
+        .expect("the output leg must still be a candidate");
+    assert!(!playback.eligible);
+    assert!(playback.sticky, "the taint is carried over, and says so");
+}
+
+// ── record accounting ─────────────────────────────────────────────────────
+
+/// Ticks exist to drive the AEC deadline, not to describe the graph. Emitting an
+/// identical record four times a second would bury the graph events the audit
+/// exists to show — but a tick that *does* change something must still be
+/// emitted, or a `Validating → Failed` transition (which only a tick can cause)
+/// would never appear in the log at all.
+#[test]
+fn an_unchanged_tick_is_suppressed_but_a_changed_one_is_not() {
+    let mut graph = Graph::new();
+    graph.app_node("music", MediaRole::StreamOutput, 100);
+    let projection = ready(graph.build());
+    let mut auditor = auditor_aec(AEC_MODULE);
+
+    assert!(auditor.observe(&projection, EventKind::Graph, 0).emit);
+    assert!(
+        !auditor.observe(&projection, EventKind::Tick, 100).emit,
+        "an identical tick record is noise"
+    );
+    assert!(
+        !auditor.observe(&projection, EventKind::Tick, 200).emit,
+        "still noise"
+    );
+
+    // The deadline expires on a tick: the state changes, so this one is emitted.
+    let outcome = auditor.observe(&projection, EventKind::Tick, TIMEOUT + 1);
+    assert!(outcome.emit);
+    assert_eq!(outcome.record.body.aec_state, "failed");
+}
+
+/// A graph event always emits, even when the decision content is identical — a
+/// suppressed graph event would erase the evidence that the graph changed at all,
+/// and "PipeWire told us something and nothing moved" is itself a finding.
+#[test]
+fn an_unchanged_graph_event_still_emits() {
+    let mut graph = Graph::new();
+    graph.app_node("music", MediaRole::StreamOutput, 100);
+    let projection = ready(graph.build());
+    let mut auditor = auditor_off();
+
+    assert!(observe(&mut auditor, &projection, 0).emit);
+    assert!(observe(&mut auditor, &projection, 1).emit);
+}
+
+/// `seq` counts every recompute, emitted or not, so a gap in the emitted
+/// sequence is visibly a suppression rather than a lost line. Without this, a
+/// reader cannot tell a quiet audit from a broken one.
+#[test]
+fn seq_counts_suppressed_recomputes_too() {
+    let mut graph = Graph::new();
+    graph.app_node("music", MediaRole::StreamOutput, 100);
+    let projection = ready(graph.build());
+    let mut auditor = auditor_off();
+
+    assert_eq!(observe(&mut auditor, &projection, 0).record.seq, 1);
+    let suppressed = auditor.observe(&projection, EventKind::Tick, 1);
+    assert!(!suppressed.emit);
+    assert_eq!(suppressed.record.seq, 2);
+    assert_eq!(observe(&mut auditor, &projection, 2).record.seq, 3);
+}
+
+/// Suppression compares against the last record actually *written*, not the last
+/// one computed. Comparing against the last computed record would let a change
+/// that appears and reverts between two ticks vanish from the log entirely,
+/// leaving a reader with a record that no longer matches the state.
+#[test]
+fn suppression_compares_against_the_last_emitted_record() {
+    let mut graph = Graph::new();
+    graph.app_node("music", MediaRole::StreamOutput, 100);
+    let with_music = ready(graph.build());
+    let empty = ready(Graph::new().build());
+    let mut auditor = auditor_off();
+
+    assert!(observe(&mut auditor, &with_music, 0).emit);
+    // A tick sees a different graph and emits.
+    assert!(auditor.observe(&empty, EventKind::Tick, 1).emit);
+    // The next tick sees the original graph again — different from what was last
+    // written, so it must be emitted.
+    assert!(auditor.observe(&with_music, EventKind::Tick, 2).emit);
+    // And now it matches the last written record.
+    assert!(!auditor.observe(&with_music, EventKind::Tick, 3).emit);
+}
+
+/// The trigger and clock are reported verbatim, which is what lets the O5 event
+/// rate be recomputed from the record stream alone rather than trusted from the
+/// summary.
+#[test]
+fn the_record_reports_its_trigger_and_clock() {
+    let projection = ready(Graph::new().build());
+    let mut auditor = auditor_off();
+
+    let outcome = auditor.observe(&projection, EventKind::Graph, 42);
+    assert_eq!(outcome.record.trigger, "graph");
+    assert_eq!(outcome.record.at_ms, 42);
+
+    let outcome = auditor.observe(&projection, EventKind::Tick, 43);
+    assert_eq!(outcome.record.trigger, "tick");
+    assert_eq!(outcome.record.at_ms, 43);
+}
+
+/// The taint view spans every media role, not just candidates. A candidate's
+/// exclusion is usually explained by taint on a node that is not itself a
+/// candidate — the sink in the middle of a forwarder — and without that the
+/// record shows the verdict but not the evidence.
+#[test]
+fn the_taint_view_covers_non_candidate_roles() {
+    let mut graph = Graph::new();
+    let call = graph.peerspeak_node("peerspeak-call", 200);
+    let sink = graph.module_node("null-sink", MediaRole::Sink, 70);
+    graph.link(call, sink);
+
+    let body = observe(&mut auditor_off(), &ready(graph.build()), 0)
+        .record
+        .body;
+    let tainted: Vec<(&str, &str)> = body
+        .taint
+        .iter()
+        .map(|row| (row.name.as_deref().unwrap_or("?"), row.reason))
+        .collect();
+
+    assert!(
+        tainted.contains(&("null-sink", "tainted-upstream")),
+        "the sink is not a candidate but its taint is what explains the row: {tainted:?}"
+    );
+    let call_name = owned_name("peerspeak-call", 200);
+    assert!(tainted.contains(&(call_name.as_str(), "peerspeak-owned")));
+}
+
+/// A record must serialise to a single line. Newlines inside a JSON Lines
+/// record would split one record into two unparseable ones — and node names come
+/// from PipeWire properties, which are attacker-adjacent free text.
+#[test]
+fn a_record_serialises_to_exactly_one_line() {
+    let mut graph = Graph::new();
+    graph.app_node("evil\nname\r\nwith breaks", MediaRole::StreamOutput, 100);
+    let projection = ready(graph.build());
+
+    let outcome = observe(&mut auditor_off(), &projection, 0);
+    let json = serde_json::to_string(&outcome.record).expect("a record must serialise");
+    assert_eq!(json.lines().count(), 1, "record split across lines: {json}");
+    assert!(
+        json.contains(r"evil\nname"),
+        "the name must survive escaped"
+    );
+}
+
+// ── O5 metrics ────────────────────────────────────────────────────────────
+
+fn sample(kind: EventKind, at_us: u64, gap_us: u64, recompute_us: u64, emit_us: u64) -> Sample {
+    Sample {
+        at_us,
+        gap_us,
+        recompute_us,
+        emit_us,
+        kind,
+    }
+}
+
+/// Bucket bounds are exclusive upper bounds, so a value exactly on a bound lands
+/// in the next bucket up. Asserted because an off-by-one here silently shifts
+/// the whole distribution the O5 conclusion rests on.
+#[test]
+fn histogram_bounds_are_exclusive_upper_bounds() {
+    let mut metrics = Metrics::default();
+    for us in [0, 49, 50, 99_999, 100_000, 1_000_000] {
+        metrics.record(sample(EventKind::Graph, 0, 1_000, us, 0));
+    }
+    let summary = metrics.summary();
+
+    assert_eq!(
+        summary.recompute_distribution,
+        vec![
+            ("<50us", 2),   // 0 and 49
+            ("<100us", 1),  // 50
+            ("<100ms", 1),  // 99_999
+            (">=100ms", 2), // 100_000 and 1_000_000
+        ]
+    );
+    assert_eq!(summary.recompute_max_us, 1_000_000);
+}
+
+/// The maximum is exact, not bucketed. O5 asks for the maximum specifically, and
+/// ">= 100 ms" is not an answer to "how bad does it get?".
+#[test]
+fn the_maximum_is_exact_not_bucketed() {
+    let mut metrics = Metrics::default();
+    metrics.record(sample(EventKind::Graph, 0, 1_000, 137, 0));
+    metrics.record(sample(EventKind::Graph, 0, 1_000, 4_211, 0));
+    metrics.record(sample(EventKind::Graph, 0, 1_000, 90, 0));
+
+    let summary = metrics.summary();
+    assert_eq!(summary.recompute_max_us, 4_211);
+    assert_eq!(summary.recompute_mean_us, Some((137 + 4_211 + 90) / 3));
+}
+
+/// Nearest-rank quantiles over the buckets.
+#[test]
+fn quantiles_use_nearest_rank_over_the_buckets() {
+    let mut metrics = Metrics::default();
+    // 99 fast samples and one very slow one: the tail must show up at p99 and
+    // nowhere earlier, which is the whole reason for reporting p99 at all.
+    for _ in 0..99 {
+        metrics.record(sample(EventKind::Graph, 0, 1_000, 10, 0));
+    }
+    metrics.record(sample(EventKind::Graph, 0, 1_000, 200_000, 0));
+
+    let summary = metrics.summary();
+    assert_eq!(summary.recompute_p50, Some("<50us"));
+    assert_eq!(summary.recompute_p90, Some("<50us"));
+    assert_eq!(summary.recompute_p99, Some("<50us"));
+    assert_eq!(summary.recompute_max_us, 200_000);
+}
+
+#[test]
+fn an_empty_histogram_reports_no_quantiles_and_no_rate() {
+    let summary = Metrics::default().summary();
+    assert_eq!(summary.recompute_p50, None);
+    assert_eq!(summary.recompute_mean_us, None);
+    assert_eq!(summary.graph_events_per_sec, None);
+    assert_eq!(summary.busy_fraction, None);
+    assert_eq!(summary.recompute_max_us, 0);
+    assert!(summary.recompute_distribution.is_empty());
+}
+
+/// Ticks are counted separately from graph events. Folding them in would inflate
+/// the measured event rate by a constant 4 Hz and hide the real graph churn —
+/// which is the number O5 is actually about.
+#[test]
+fn ticks_do_not_count_toward_the_graph_event_rate() {
+    let mut metrics = Metrics::default();
+    // Two graph events one second apart, with ticks in between.
+    metrics.record(sample(EventKind::Graph, 0, 0, 100, 0));
+    for i in 1..4 {
+        metrics.record(sample(EventKind::Tick, i * 250_000, 250_000, 100, 0));
+    }
+    metrics.record(sample(EventKind::Graph, 1_000_000, 250_000, 100, 0));
+
+    let summary = metrics.summary();
+    assert_eq!(summary.graph_events, 2);
+    assert_eq!(summary.tick_events, 3);
+    // Span runs to the last sample's completion: 1_000_000 + 100 µs.
+    assert_eq!(summary.span_us, 1_000_100);
+    assert_eq!(summary.graph_events_per_sec, Some(2.0));
+}
+
+/// The queueing proxy: an event beginning within the threshold of the previous
+/// sample's completion was almost certainly already waiting. The first sample is
+/// never counted — it has no predecessor to have queued behind, and counting it
+/// would put a phantom backlog in every run.
+#[test]
+fn the_queueing_proxy_counts_back_to_back_events_only() {
+    let mut metrics = Metrics::default();
+    metrics.record(sample(EventKind::Graph, 0, 0, 100, 0));
+    metrics.record(sample(EventKind::Graph, 100, QUEUE_THRESHOLD_US, 100, 0));
+    metrics.record(sample(
+        EventKind::Graph,
+        200,
+        QUEUE_THRESHOLD_US + 1,
+        100,
+        0,
+    ));
+    metrics.record(sample(EventKind::Graph, 300, 0, 100, 0));
+
+    let summary = metrics.summary();
+    assert_eq!(
+        summary.queued_events, 2,
+        "exactly the two within the threshold, never the first sample"
+    );
+    assert_eq!(summary.queue_threshold_us, QUEUE_THRESHOLD_US);
+}
+
+/// Emission cost is tracked separately from recompute cost, and a suppressed
+/// record contributes neither an emitted-record count nor an emit sample —
+/// otherwise the logging distribution would be diluted by every tick that wrote
+/// nothing.
+#[test]
+fn emission_cost_is_tracked_separately_from_recompute() {
+    let mut metrics = Metrics::default();
+    metrics.record(sample(EventKind::Graph, 0, 0, 300, 80));
+    metrics.record(sample(EventKind::Tick, 1_000, 900, 200, 0));
+    metrics.record(sample(EventKind::Graph, 2_000, 900, 400, 120));
+
+    let summary = metrics.summary();
+    assert_eq!(summary.emitted_records, 2);
+    assert_eq!(summary.emit_max_us, 120);
+    assert_eq!(summary.emit_mean_us, Some(100));
+    assert_eq!(
+        summary.emit_distribution,
+        vec![("<100us", 1), ("<250us", 1)]
+    );
+    // Busy time is recompute *and* logging: 300+80+200+400+120.
+    assert_eq!(summary.busy_us, 1_100);
+}
+
+/// The busy fraction needs no inference, unlike the queueing proxy, so it is the
+/// number the O5 verdict should lean on.
+#[test]
+fn the_busy_fraction_is_the_share_of_wall_time_spent_working() {
+    let mut metrics = Metrics::default();
+    metrics.record(sample(EventKind::Graph, 0, 0, 100, 0));
+    // Ends at 1_000_000 + 900 → a span of 1_000_900 µs with 1_000 µs of work.
+    metrics.record(sample(EventKind::Graph, 1_000_000, 999_900, 900, 0));
+
+    let summary = metrics.summary();
+    assert_eq!(summary.busy_us, 1_000);
+    assert_eq!(summary.span_us, 1_000_900);
+    assert_eq!(summary.busy_fraction, Some(0.001));
+}
+
+/// Bucket labels and bounds must stay parallel, or the distribution mislabels
+/// itself — a silent failure that would misreport every O5 result.
+#[test]
+fn bucket_labels_cover_every_bound_plus_overflow() {
+    assert_eq!(
+        BUCKET_LABELS.len(),
+        super::metrics::BUCKET_BOUNDS_US.len() + 1
+    );
+}

src/host/mod.rs

@@ -1,8 +1,12 @@
+pub mod aec;
 pub mod audio;
+pub mod audit;
 mod capture;
+mod observer;
 mod pipeline;
 mod quality;
 mod serve;
+pub mod taint;
 mod wayland;
 mod x11;
 
@@ -74,6 +78,12 @@ pub async fn run(opts: HostOpts) -> Result<()> {
 
     let cancel = signal::install_ctrl_c();
 
+    // Phase 5 dry-run audit, off unless `PIXELPASS_AUDIO_AUDIT=1`. Read-only:
+    // it observes the graph and logs what phases 2–4 conclude, creating no
+    // links. Bound to a name so the handle lives as long as the run — dropping
+    // it stops the observer thread and flushes the final O5 summary.
+    let _audio_audit = audit::run::spawn_if_enabled()?;
+
     let endpoint = endpoint::bind(opts.relay.as_deref()).await?;
 
     // Relay-only ticket: wait for the home relay to connect, then keep only

src/host/observer/classify.rs

@@ -0,0 +1,228 @@
+//! The `session_device` classifier — pure, no PipeWire.
+//!
+//! `NodeProps::session_device` (see [`super::super::taint::snapshot`]) is a
+//! **positive high-confidence** claim that a node is a passive hardware
+//! terminal: a real sound card's sink or source that terminates audio rather
+//! than forwarding it. Setting it *removes* two protections at once — the
+//! node's coarse owner keys and its ability to trip the fail-closed backstop
+//! — so a false positive is a **leak**, and the whole classifier is shaped so
+//! that anything less than a positive identification resolves to `false`.
+//!
+//! The observer (phase 3) owes this classification; the adapter must never
+//! stuff a raw property through. Two facts from the design (v3.4 §6.1.1,
+//! Codex rounds 2–4) drive the shape here:
+//!
+//! - `device.id` / `device.api` describe *which* Device a node belongs to and
+//!   *how* that Device is reached — **neither promises the node passively
+//!   terminates audio.** A filter chain associated with a card satisfies
+//!   both. So the discriminator is `factory.name` on an **allowlist** of
+//!   real hardware-PCM factories, never a substring or a denylist: an unknown
+//!   factory is not a device.
+//! - The backing Device must actually have been **bound and resolved**. A node
+//!   that claims a `device.id` whose Device's properties we do not hold is
+//!   **withheld**, not admitted with a provisional `false` — a provisional
+//!   `false` during the not-ready window fuses sink and mic on the shared
+//!   session client and that fusion can persist as sticky over-exclusion
+//!   (round-3 finding 3).
+//!
+//! **Round 8 (v3.5 §6.7 decision 4): the Device is the authority on
+//! `device.api` and `alsa.driver_name`.** Both are absent from the Node
+//! *global* and both are present on the **bound Device**'s `info` props
+//! (measured 2026-07-25). Reading them from the Device closes the phase-3
+//! review's owed fix: on PipeWire ≥ 1.2.6 with WirePlumber < 0.5.13 the driver
+//! name is not copied onto the node, and the fail-closed "absent driver ⇒ not
+//! a session device" rule would over-exclude real sound cards. `factory.name`
+//! exists only on the node, which is why the node bind is required regardless.
+
+use crate::host::taint::snapshot::GlobalId;
+
+/// Factory names that positively identify a passive hardware-PCM terminal.
+///
+/// **An allowlist, deliberately.** Membership *removes* protections, so the
+/// safe error direction is to leave a genuine-but-unlisted device off the
+/// list (it merely keeps its owner keys — over-exclusion, no echo). Adding a
+/// backend here is a security-relevant change and wants the same measurement
+/// the ALSA entries got (snapshot.rs `session_device` contract: the target
+/// box's five ALSA nodes carry `factory.name=api.alsa.pcm.{sink,source}`; the
+/// three `support.null-audio-sink` nodes carry neither).
+///
+/// `support.null-audio-sink`, `*.loopback`, and any filter factory are
+/// intentionally **absent**: those forward audio, which is exactly the shape
+/// this feature must be able to exclude.
+///
+/// ⚠️ **ALSA only, and only these two, because they are the only factories
+/// measured on the target box.** BlueZ was previously listed here as
+/// `api.bluez5.pcm.{sink,source}` — those are invented; the real BlueZ
+/// terminals are `api.bluez5.media.{sink,source}` with profile aliases
+/// (Codex phase-3 review, finding 5). Rather than allowlist an unmeasured
+/// guess, BlueZ is left off entirely: a real Bluetooth sink then keeps its
+/// owner keys (over-exclusion — safe). Add BlueZ back only with a *measured*
+/// factory name and a fixture.
+const HARDWARE_PCM_FACTORIES: &[&str] = &[
+    // ALSA — measured on the target box.
+    "api.alsa.pcm.sink",
+    "api.alsa.pcm.source",
+];
+
+/// The `device.api` every entry in [`HARDWARE_PCM_FACTORIES`] belongs to.
+/// A single value rather than a list, because the allowlist is ALSA-only;
+/// this constant is the thing to change when that stops being true.
+const HARDWARE_PCM_API: &str = "alsa";
+
+/// ALSA drivers that expose a hardware-PCM `factory.name` but are **not**
+/// passive terminals — audio written in reappears on their capture side
+/// through a path the PipeWire Link graph cannot see, so classifying them
+/// `session_device` (which drops owner keys and the fail-closed backstop)
+/// would let tainted audio loop back untainted (Codex phase-3 review,
+/// finding 2). `factory.name` alone cannot distinguish these from a real
+/// card — `snd_aloop` presents as `api.alsa.pcm.{sink,source}` exactly like
+/// `snd_hda_intel` — so a real ALSA terminal must present an `alsa.driver_name`
+/// that is **present and not on this denylist**; a missing driver fails closed
+/// (see [`classify`]). `snd_dummy` is intentionally absent: it is virtual but
+/// does not couple playback to capture, so it is not a loopback hazard.
+const NON_TERMINAL_ALSA_DRIVERS: &[&str] = &["snd_aloop"];
+
+/// The node-side properties the classifier reads, exactly as the adapter
+/// parsed them off the **bound Node's `info`** (never off the registry
+/// global — v3.5 §6.7). Kept separate from
+/// [`super::super::taint::snapshot::NodeProps`] because these feed the
+/// *decision* whose output is the `session_device` field — they are inputs,
+/// not part of the graph the engine reasons over.
+#[derive(Clone, Debug, Default, PartialEq, Eq)]
+pub struct DeviceClaim {
+    /// `device.id` — the Device this node belongs to, if any. Absent on
+    /// `Stream/*` nodes, which is exactly why their absence means "not a
+    /// device", not "unknown".
+    pub device_id: Option<GlobalId>,
+    /// `device.api` **as copied onto the node**, when it is — the access API
+    /// of that Device (e.g. `alsa`, `bluez5`). Its mere presence is **not**
+    /// sufficient (a card-associated filter has it too); required only as a
+    /// corroborating signal alongside the factory allowlist. The
+    /// authoritative copy is [`DeviceProps::device_api`]; this is the
+    /// fallback.
+    pub device_api: Option<String>,
+    /// `factory.name` — the discriminator. Only an allowlisted hardware-PCM
+    /// factory earns `session_device`.
+    pub factory_name: Option<String>,
+    /// `alsa.driver_name` — the kernel driver behind an ALSA node (e.g.
+    /// `snd_hda_intel`, `snd_usb_audio`, `snd_aloop`). Needed because the
+    /// factory allowlist cannot tell a real card from a loopback driver that
+    /// shares the same factory. `session_device` requires this to be
+    /// **present and not** on [`NON_TERMINAL_ALSA_DRIVERS`]; a driver on the
+    /// denylist, or an absent value, both fail closed (see [`classify`]).
+    /// Frequently absent here — PipeWire ≥ 1.2.6 with WirePlumber < 0.5.13
+    /// does not copy `alsa.*` onto the node — which is why the authoritative
+    /// copy is [`DeviceProps::alsa_driver_name`] and this is only the
+    /// fallback.
+    pub alsa_driver_name: Option<String>,
+}
+
+/// The **bound Device's** `info` properties — the authoritative half of the
+/// `session_device` decision (v3.5 §6.7 decision 4).
+///
+/// Absent from the Device *registry global* exactly as the node's properties
+/// are absent from the Node global; both are recovered by binding. A node
+/// claiming a `device.id` is withheld until this struct exists for that
+/// Device (see [`Classification::Withhold`]).
+#[derive(Clone, Debug, Default, PartialEq, Eq)]
+pub struct DeviceProps {
+    /// `device.api` on the Device — `alsa`, `bluez5`, `v4l2`, …
+    pub device_api: Option<String>,
+    /// `alsa.driver_name` on the Device — the kernel driver behind the card,
+    /// authoritative regardless of whether the session manager copied it onto
+    /// the node.
+    pub alsa_driver_name: Option<String>,
+}
+
+/// The outcome of classifying one node's device claim.
+#[derive(Clone, Copy, PartialEq, Eq, Debug)]
+pub enum Classification {
+    /// No `device.id` — a `Stream/*` node. Admit with `session_device=false`.
+    NotADevice,
+    /// A `device.id` is claimed but the backing Device's properties are not
+    /// held: never observed, its bind still outstanding, or its global id
+    /// ambiguously shared by two live Devices. **Withhold the node and keep
+    /// the readiness epoch not-ready**; re-classify when the Device resolves.
+    Withhold { device_id: GlobalId },
+    /// Positively a passive hardware terminal. Admit with
+    /// `session_device=true`.
+    SessionDevice,
+    /// Backed by a *resolved* Device but not a hardware-PCM terminal — a
+    /// filter or virtual node on a card, an unknown factory, or a Device with
+    /// no `device.api`. Admit with `session_device=false` (fail closed).
+    NotSessionDevice,
+}
+
+/// Classify a node's device claim against its backing Device.
+///
+/// `device` is the bound Device's properties, and `None` means the claim is
+/// **unresolved** — never observed, bind outstanding, or an ambiguous
+/// recycled id. It is only consulted when a `device_id` is present. Pure: the
+/// model looks the Device up, and the I/O of *binding* it lives in the
+/// adapter.
+///
+/// Where the two sides disagree the rule is deliberately asymmetric, and
+/// safety picks the direction (v3.5 §6.7 decision 4):
+///
+/// - **Presence: the Device wins, the node is the fallback.** That is what
+///   recovers a real card whose node was never given `alsa.driver_name`.
+/// - **The denylist is a union.** If *either* side names a non-terminal
+///   driver the node is not a session device. A disagreement here is not
+///   expected on any measured configuration, and treating it as "the Device
+///   says it is fine" would be the one reading that can leak.
+pub fn classify(claim: &DeviceClaim, device: Option<&DeviceProps>) -> Classification {
+    let Some(device_id) = claim.device_id else {
+        // No backing Device: a stream. Not withheld, not a device.
+        return Classification::NotADevice;
+    };
+    let Some(device) = device else {
+        // Backed by a Device we have not resolved — the one case that blocks
+        // readiness. A provisional answer here is the leak the contract
+        // forbids.
+        return Classification::Withhold { device_id };
+    };
+    let on_factory_allowlist = claim
+        .factory_name
+        .as_deref()
+        .is_some_and(|f| HARDWARE_PCM_FACTORIES.contains(&f));
+    // A **present, non-denied** ALSA driver is required — absence fails closed
+    // (Codex phase-3 re-review). The factory allowlist cannot tell a real card
+    // from `snd_aloop`, which presents the same `api.alsa.pcm.*` factory, so a
+    // *missing* value must not be read as "not a loopback". Round 8 makes the
+    // bound Device the primary source, so a real card is no longer
+    // over-excluded merely because the session manager did not copy `alsa.*`
+    // onto its node.
+    let driver = device
+        .alsa_driver_name
+        .as_deref()
+        .or(claim.alsa_driver_name.as_deref());
+    let driver_denied = [
+        device.alsa_driver_name.as_deref(),
+        claim.alsa_driver_name.as_deref(),
+    ]
+    .into_iter()
+    .flatten()
+    .any(|d| NON_TERMINAL_ALSA_DRIVERS.contains(&d));
+    let driver_ok = driver.is_some() && !driver_denied;
+    // The API must positively be the one the factory allowlist is written
+    // for, not merely present (Codex phase-3r review, finding 3). "Present"
+    // admitted `device.api=v4l2` alongside `factory.name=api.alsa.pcm.sink`
+    // — a contradiction no truthful configuration produces, which is exactly
+    // why it should be read as an observation gone wrong rather than as
+    // corroboration. Disagreement between the two sides fails closed for the
+    // same reason. ⚠️ Tied to [`HARDWARE_PCM_FACTORIES`] being ALSA-only:
+    // adding a BlueZ factory means allowing `bluez5` here too.
+    let api_ok = match (device.device_api.as_deref(), claim.device_api.as_deref()) {
+        (Some(from_device), Some(from_node)) if from_device != from_node => false,
+        (Some(api), _) | (None, Some(api)) => api == HARDWARE_PCM_API,
+        (None, None) => false,
+    };
+    let is_hardware_pcm = api_ok && on_factory_allowlist && driver_ok;
+    if is_hardware_pcm {
+        Classification::SessionDevice
+    } else {
+        // Resolved, but not positively a terminal: fail closed to false so
+        // the node keeps its owner keys and its backstop.
+        Classification::NotSessionDevice
+    }
+}

src/host/observer/mod.rs

@@ -0,0 +1,760 @@
+//! The registry observer's **pure core** (impl plan §4, phases 3 and 3r).
+//!
+//! This is my half of the phase-3 split: a reducer that folds a stream of
+//! typed [`RegEvent`]s into a live model of the PipeWire graph and projects
+//! the [`GraphSnapshot`] + context the taint engine (phase 2) consumes. **No
+//! PipeWire types appear here** — the I/O adapter (Codex's half) translates
+//! live registry callbacks, binds, `/proc` reads, and the `core.sync`/`done`
+//! round-trip into these events and feeds them in. Every test in this module
+//! builds the event stream by hand.
+//!
+//! ## 🔴 Round 8 (v3.5 §6.7): the global is an INDEX, not a source of truth
+//!
+//! Phase 3 shipped reading node properties off the registry `global` event.
+//! The registry announces only a fixed 13-key subset for a Node, and **eight
+//! properties this feature depends on are never among them** — they read as
+//! absent rather than failing, so the engine was silently, permanently
+//! starved of both its primary taint root and every strong owner key (the
+//! phase-5 gate failure, F1/F2). The rule that replaces it:
+//!
+//! > A node's properties come from a **bind**, never from the global. The
+//! > global tells us an object exists, its id and its serial. Everything
+//! > else — including `node.name` and `media.class`, so there is exactly one
+//! > source — arrives on [`RegEvent::NodeInfo`]. Same for `Device`
+//! > ([`RegEvent::DeviceInfo`]).
+//!
+//! Consequences visible in this file: a Node is admitted to the snapshot
+//! **only** once its `info` has arrived (until then it is withheld and is a
+//! readiness obligation); a Device resolves a node's claim only once *its*
+//! `info` has arrived; and `info` may fire again for the lifetime of the
+//! object, so [`RegEvent::NodeInfo`] is both the first resolution and every
+//! later property change (v3.5 §6.7 decisions 1–4).
+//!
+//! Three things this core is shaped to get right, each an exit-gate row:
+//!
+//! - **Removal by recycled id.** `global_remove` names only a 32-bit global
+//!   id, and those recycle. The model keeps an insertion-ordered index per id
+//!   so a removal accounts for the *oldest* generation first, and the
+//!   snapshot projection treats any id still claimed by two live objects as
+//!   [`IdLookup::Ambiguous`] — fail closed (v3.4 §6.1.3). Everything the
+//!   model *owns* is keyed by never-recycled `object.serial`; ids are only
+//!   ever a lookup.
+//! - **The readiness epoch.** `graph_ready` is false until the initial graph
+//!   is fully observed: the server has synced **and** no binds/withheld nodes
+//!   remain outstanding. A bounded timeout makes it fail closed. It gates
+//!   sticky *retirement* only; withholding after completion is per-object.
+//! - **Withholding on unresolved input.** A node with no `info` yet, or one
+//!   claiming a `device.id` whose Device we have not resolved, is held out of
+//!   the snapshot entirely rather than admitted with provisional ownership
+//!   (see [`classify`]).
+//!
+//! **Three accepted limitations, all low-reachability, owed to a later
+//! hardening round:**
+//!
+//! - *A Link dropped for a missing `object.serial`/props is unrepresented.*
+//!   The adapter drops such a global before it reaches [`RegistryModel`], so
+//!   readiness can reach `Complete` while permanently omitting that Link — an
+//!   invisible edge that could hide tainted ancestry. **Not reachable in
+//!   practice:** PipeWire's native protocol defines `object.serial` as the
+//!   unique identity every global carries, so a Link without one requires a
+//!   protocol/server failure, not ordinary churn. (The live gate is
+//!   consistent with this but does not *prove* it — it only counts Links the
+//!   strict parser already admitted.) A full fix needs a pure
+//!   "required-observation-failed" token that holds readiness false; deferred
+//!   rather than built for a case that does not occur.
+//! - *Removal generation ordering assumes no removal is silently lost.* On a
+//!   recycled id with two live claimants, [`Self::on_removed`] retires the
+//!   oldest generation first; if the *first* generation's removal was never
+//!   delivered, a later removal is misattributed. PipeWire's registry does not
+//!   silently drop `global_remove`, so this needs callback loss to trigger.
+//!   The snapshot treats the two-claimant window as [`IdLookup::Ambiguous`]
+//!   (fail closed) meanwhile.
+//! - *An unresolvable bind takes the whole graph down, not just its node*
+//!   (v3.5 §6.7 decision 3). A node whose `info` never arrives keeps
+//!   readiness false until the deadline, then sticky-[`Readiness::TimedOut`]
+//!   — no fan-out at all, identical to a never-resolving Link bind. Per-node
+//!   quarantine (that node ineligible **and** taint-bearing, the rest of the
+//!   graph still working) is strictly better and is deferred because it is a
+//!   new concept in the *pure engine*, not a fix to the observer.
+
+#![allow(dead_code)] // Wired by the phase-3 adapter (Codex's half) and consumed by later phases.
+
+pub mod adapter;
+pub mod classify;
+pub mod pulse_pid;
+
+#[cfg(test)]
+mod tests;
+
+use crate::host::taint::snapshot::{
+    ClientSnapshot, GlobalId, GraphSnapshot, LinkSnapshot, MediaRole, NodeProps, NodeSnapshot,
+    PortSnapshot, Serial,
+};
+use classify::{Classification, DeviceClaim, DeviceProps};
+use std::collections::{BTreeMap, VecDeque};
+
+/// A monotonic millisecond clock value, supplied by the adapter via
+/// [`RegEvent::Tick`]. Kept as a bare integer rather than
+/// [`std::time::Instant`] so the readiness timeout is deterministic in tests.
+pub type Millis = u64;
+
+/// A Node's **bound `info` properties** — the sole source of node properties
+/// (v3.5 §6.7), delivered by [`RegEvent::NodeInfo`].
+///
+/// This carries no identity: the serial names the node on the event and the
+/// global id was recorded by [`RegEvent::NodeAdded`], so the adapter cannot
+/// contradict the index it already published. `session_device` inside
+/// [`NodeObservation::props`] is left at its `false` default; the model
+/// overwrites it from the [`classify`] result at projection time, once the
+/// backing Device (if any) is resolved.
+#[derive(Clone, Debug, PartialEq, Eq)]
+pub struct NodeObservation {
+    pub name: Option<String>,
+    pub role: MediaRole,
+    pub props: NodeProps,
+    pub device_claim: DeviceClaim,
+}
+
+/// The four endpoint references a Link carries. Node endpoints are required —
+/// a Link with unknown nodes is useless — so this whole struct is what the
+/// adapter must resolve (from the global's props if present, else by binding
+/// `LinkInfoRef`, the correctness path) before a Link enters the snapshot.
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+pub struct LinkEndpoints {
+    pub output_node: GlobalId,
+    pub input_node: GlobalId,
+    pub output_port: Option<GlobalId>,
+    pub input_port: Option<GlobalId>,
+}
+
+/// A typed observation of the live graph. The adapter produces these; the
+/// model consumes them in [`RegistryModel::apply`].
+#[derive(Clone, Debug, PartialEq, Eq)]
+pub enum RegEvent {
+    /// A Node global appeared. **Index only** — the global's properties are a
+    /// filtered subset and are not read (v3.5 §6.7). The node is withheld
+    /// from the snapshot and is a readiness obligation until its
+    /// [`RegEvent::NodeInfo`] arrives.
+    NodeAdded { serial: Serial, id: GlobalId },
+    /// A bound Node's `info` properties. **Both** the first resolution and
+    /// every later `PROPS` change for the node's lifetime — the model tells
+    /// them apart, so the adapter holds no per-node "have I seen info yet?"
+    /// state to get wrong. An `info` for a serial we do not hold (a node
+    /// already removed) is ignored.
+    NodeInfo {
+        serial: Serial,
+        observation: NodeObservation,
+    },
+    /// A Port global appeared.
+    PortAdded(PortSnapshot),
+    /// A Client global appeared. Feeds pulse-PID derivation via `sec_pid`.
+    ClientAdded(ClientSnapshot),
+    /// A Device global appeared. Index only, exactly as for a Node: it does
+    /// not resolve anything until [`RegEvent::DeviceInfo`] arrives.
+    DeviceAdded { serial: Serial, id: GlobalId },
+    /// A bound Device's `info` properties — the **authoritative** source of
+    /// `device.api` and `alsa.driver_name` (v3.5 §6.7 decision 4). Resolves
+    /// every node withheld on this Device's id.
+    DeviceInfo { serial: Serial, props: DeviceProps },
+    /// A Link global appeared. `endpoints` is `Some` when the global carried
+    /// them (the optimisation) and `None` when the adapter must bind to learn
+    /// them (the correctness path) — the latter is an outstanding obligation
+    /// until a matching [`RegEvent::LinkEndpointsResolved`] arrives.
+    ///
+    /// Unlike Nodes and Devices, Link endpoint props **are** announced on the
+    /// global (measured, phase-5 results F1), so this asymmetry is real and
+    /// deliberate.
+    LinkAdded {
+        serial: Serial,
+        id: GlobalId,
+        endpoints: Option<LinkEndpoints>,
+    },
+    /// The bind-`LinkInfoRef` fallback resolved a Link's endpoints.
+    LinkEndpointsResolved {
+        serial: Serial,
+        endpoints: LinkEndpoints,
+    },
+    /// The adapter read `/proc/<pid>/comm` (`None` = the read failed / the
+    /// process is gone). Validates the pulse-PID candidate.
+    ProcCommProbed { pid: u32, comm: Option<String> },
+    /// Any global was removed. Only its 32-bit id is known.
+    Removed { id: GlobalId },
+    /// A `core.sync()` issued after the initial enumeration completed its
+    /// round-trip (`done`). One half of readiness; the other is that no
+    /// binds/withheld nodes are still outstanding.
+    ServerSynced,
+    /// A monotonic clock sample. Drives the readiness timeout only.
+    Tick { now: Millis },
+}
+
+/// What kind of observation drove a projection.
+///
+/// Derived from the event itself ([`RegEvent::kind`]) rather than passed
+/// alongside it, so a consumer's view of "was this a real graph change?" cannot
+/// disagree with what the model was actually fed. The distinction matters to the
+/// phase-5 audit twice over: ticks arrive at a constant rate and would inflate
+/// any measured graph-event rate, and a record that is identical to the previous
+/// one is worth suppressing on a tick but never on a graph event.
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+pub enum EventKind {
+    /// A registry observation: an add, a removal, a bind resolution, a `/proc`
+    /// probe, or the server sync.
+    Graph,
+    /// The periodic clock sample. Carries no graph information; it exists so the
+    /// readiness timeout and the AEC validation deadline have a clock.
+    Tick,
+}
+
+impl EventKind {
+    pub fn code(self) -> &'static str {
+        match self {
+            Self::Graph => "graph",
+            Self::Tick => "tick",
+        }
+    }
+}
+
+impl RegEvent {
+    pub fn kind(&self) -> EventKind {
+        match self {
+            Self::Tick { .. } => EventKind::Tick,
+            _ => EventKind::Graph,
+        }
+    }
+}
+
+/// Whether an applied event could have changed the projection.
+///
+/// The suppression rule of v3.5 §6.7 decision 2, in the one place that can
+/// enforce it: **a property update may be dropped only when the resulting
+/// [`Projection`] is identical to the current one.** The projection is a pure
+/// function of model state, so "state provably unchanged" *is* "projection
+/// identical" — which is what [`Outcome::Suppressed`] means and why the check
+/// is a cheap field comparison rather than building and diffing two snapshots.
+///
+/// Anything looser (dropping updates that do change state) breaks phase 4's
+/// no-coalescing contract, which needs to see the empty gap between an AEC
+/// module unload and a reload that reuses the index. Anything stricter
+/// (publishing on every `info`, including the state-only changes PipeWire
+/// emits constantly) inflates the O5 event rate with non-events.
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+pub enum Outcome {
+    /// Model state may have changed; the caller must publish the projection.
+    Applied,
+    /// Model state provably did not change; publishing is optional and the
+    /// adapter skips it.
+    Suppressed,
+}
+
+/// Which slot in the id index a live object occupies. `global_remove` gives
+/// only the id, so the index remembers what each id currently holds. Every
+/// slot names its object by never-recycled serial.
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+enum Slot {
+    Node(Serial),
+    Port(Serial),
+    Link(Serial),
+    Client(Serial),
+    Device(Serial),
+}
+
+/// The readiness epoch. A one-time transition out of [`Readiness::Waiting`];
+/// both terminal states are sticky (a completed graph is not un-completed by
+/// later per-object withholding, and a timed-out observer stays fail-closed
+/// for its lifetime).
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+pub enum Readiness {
+    /// The initial enumeration is still in flight.
+    Waiting,
+    /// The initial enumeration finished at least once (server synced with no
+    /// obligations then outstanding). **Sticky** — later per-object
+    /// withholding does not revert it. Note this is *not* the same as
+    /// [`RegistryModel::graph_ready`], which additionally requires no *current*
+    /// obligation (Codex finding 1); `Complete` only records that the epoch
+    /// was reached.
+    Complete,
+    /// The bounded deadline passed with obligations outstanding.
+    /// `graph_ready` stays false — fail closed.
+    TimedOut,
+}
+
+/// The pure handoff to the taint engine: a coherent [`GraphSnapshot`] plus the
+/// two context fields phase 3 owns. The caller merges these into
+/// [`crate::host::taint::ExclusionCtx`] alongside `aec_module_id` (phase 4)
+/// and `pixelpass_owned` (pixelpass's own tracking).
+#[derive(Clone, Debug, PartialEq, Eq)]
+pub struct Projection {
+    pub snapshot: GraphSnapshot,
+    pub pipewire_pulse_pid: Option<u32>,
+    pub graph_ready: bool,
+    /// The sticky readiness epoch behind `graph_ready`. Carried so a consumer
+    /// can tell the three not-ready causes apart — enumeration still in flight
+    /// ([`Readiness::Waiting`]), a fail-closed timeout ([`Readiness::TimedOut`]),
+    /// or a completed epoch momentarily blocked on a current obligation
+    /// ([`Readiness::Complete`] with `graph_ready == false`). `graph_ready`
+    /// alone collapses all three into "no". The phase-5 audit reports it as the
+    /// epoch column; nothing gates on it.
+    pub readiness: Readiness,
+}
+
+/// A live Node: its global id (for link endpoint lookup) plus its bound
+/// properties once they arrive.
+#[derive(Clone, Debug, PartialEq, Eq)]
+struct NodeEntry {
+    id: GlobalId,
+    /// `None` while the bind is outstanding — withheld from the snapshot and
+    /// an outstanding readiness obligation (v3.5 §6.7 decision 3).
+    obs: Option<NodeObservation>,
+}
+
+/// A live Device: its global id plus its bound properties once they arrive.
+#[derive(Clone, Debug, PartialEq, Eq)]
+struct DeviceEntry {
+    id: GlobalId,
+    /// `None` while the bind is outstanding. A node claiming this Device
+    /// stays withheld until it is `Some` — the Device's `device.api` and
+    /// `alsa.driver_name` are the authoritative inputs to `session_device`
+    /// (v3.5 §6.7 decision 4), so classifying without them would be the same
+    /// provisional answer the contract forbids.
+    props: Option<DeviceProps>,
+}
+
+/// The live model. Folds [`RegEvent`]s; project with [`RegistryModel::project`].
+#[derive(Clone, Debug)]
+pub struct RegistryModel {
+    /// **Every** live Node, keyed by serial — admitted or withheld. Admission
+    /// is decided at projection time from the entry's own state, so there is
+    /// no admitted/withheld pair of maps to drift apart.
+    nodes: BTreeMap<Serial, NodeEntry>,
+    /// Every live Device, keyed by serial.
+    devices: BTreeMap<Serial, DeviceEntry>,
+    ports: BTreeMap<Serial, PortSnapshot>,
+    links: BTreeMap<Serial, LinkSnapshot>,
+    clients: BTreeMap<Serial, ClientSnapshot>,
+
+    /// Links whose endpoints the adapter is still binding; the id is kept so
+    /// removal and resolution can find them.
+    pending_links: BTreeMap<Serial, GlobalId>,
+
+    /// Insertion-ordered holders of each live global id. `global_remove`
+    /// accounts for the oldest generation first (v3.4 §6.1.3).
+    live_ids: BTreeMap<GlobalId, VecDeque<Slot>>,
+
+    /// `/proc/<pid>/comm` reads keyed by pid, for pulse-PID validation.
+    probed_comm: BTreeMap<u32, Option<String>>,
+
+    server_synced: bool,
+    readiness: Readiness,
+    deadline: Millis,
+    last_now: Millis,
+}
+
+impl RegistryModel {
+    /// `now` seeds the clock; `timeout` is the readiness budget. The deadline
+    /// is `now + timeout`; a [`RegEvent::Tick`] at or past it while still
+    /// [`Readiness::Waiting`] fails the epoch closed.
+    pub fn new(now: Millis, timeout: Millis) -> Self {
+        Self {
+            nodes: BTreeMap::new(),
+            devices: BTreeMap::new(),
+            ports: BTreeMap::new(),
+            links: BTreeMap::new(),
+            clients: BTreeMap::new(),
+            pending_links: BTreeMap::new(),
+            live_ids: BTreeMap::new(),
+            probed_comm: BTreeMap::new(),
+            server_synced: false,
+            readiness: Readiness::Waiting,
+            deadline: now.saturating_add(timeout),
+            last_now: now,
+        }
+    }
+
+    pub fn readiness(&self) -> Readiness {
+        self.readiness
+    }
+
+    /// Whether the graph is trustworthy enough to make eligibility and sticky
+    /// **retirement** decisions right now.
+    ///
+    /// This is **dynamic**, not the sticky [`Readiness::Complete`] flag: it is
+    /// true only when the initial enumeration has completed **and** there are
+    /// no current obligations outstanding (a node whose bind is outstanding, a
+    /// node withheld on an unresolved Device, or a Link still being bound).
+    /// The distinction is the fix for Codex phase-3 review finding 1: a Link
+    /// whose endpoints are still resolving is an **invisible edge** — it is
+    /// absent from the snapshot, not merely dangling — so a decision made
+    /// while one exists can miss real tainted ancestry and wrongly report a
+    /// candidate eligible. Unresolved ancestry ⇒ fail closed is the governing
+    /// invariant (v3.4 §6.1), and round 8 adds the far more common case: an
+    /// unbound node is an invisible *vertex*, which hides everything the edge
+    /// case hides and its ownership besides.
+    ///
+    /// [`Readiness::Complete`] stays sticky (it records that the initial
+    /// enumeration happened, for logging and to distinguish "not started" from
+    /// "momentarily churning"); `graph_ready` layers the dynamic obligation
+    /// check on top. Downstream (phase 6) may debounce the brief blips a
+    /// normal bind causes; the observer's job is to report the truth.
+    pub fn graph_ready(&self) -> bool {
+        matches!(self.readiness, Readiness::Complete) && !self.obligations_outstanding()
+    }
+
+    /// The pulse-PID candidate the adapter should be probing (`None` = no
+    /// repeated `sec_pid`, nothing to probe). Exposed so the adapter re-probes
+    /// only when the candidate changes.
+    pub fn pulse_pid_candidate(&self) -> Option<u32> {
+        let clients: Vec<ClientSnapshot> = self.clients.values().cloned().collect();
+        pulse_pid::candidate(&clients)
+    }
+
+    /// Fold one observation into the model. The returned [`Outcome`] tells the
+    /// caller whether the projection can have changed; see [`Outcome`] for why
+    /// that is the only sound place to enforce the suppression rule.
+    pub fn apply(&mut self, event: RegEvent) -> Outcome {
+        match event {
+            RegEvent::NodeAdded { serial, id } => {
+                self.push_id(id, Slot::Node(serial));
+                self.nodes.insert(serial, NodeEntry { id, obs: None });
+                // A node awaiting its bind is a fresh obligation, so this can
+                // only ever *hold* readiness, never complete it — but the
+                // re-check is cheap and keeps the invariant local.
+                self.maybe_complete();
+                Outcome::Applied
+            }
+            RegEvent::NodeInfo {
+                serial,
+                observation,
+            } => self.on_node_info(serial, observation),
+            RegEvent::PortAdded(port) => {
+                self.push_id(port.id, Slot::Port(port.serial));
+                self.ports.insert(port.serial, port);
+                Outcome::Applied
+            }
+            RegEvent::ClientAdded(client) => {
+                self.push_id(client.id, Slot::Client(client.serial));
+                self.clients.insert(client.serial, client);
+                // A new client can change the pulse candidate; the adapter
+                // learns that via `pulse_pid_candidate`. No readiness effect.
+                Outcome::Applied
+            }
+            RegEvent::DeviceAdded { serial, id } => {
+                self.push_id(id, Slot::Device(serial));
+                self.devices.insert(serial, DeviceEntry { id, props: None });
+                self.maybe_complete();
+                Outcome::Applied
+            }
+            RegEvent::DeviceInfo { serial, props } => self.on_device_info(serial, props),
+            RegEvent::LinkAdded {
+                serial,
+                id,
+                endpoints,
+            } => {
+                self.on_link_added(serial, id, endpoints);
+                Outcome::Applied
+            }
+            RegEvent::LinkEndpointsResolved { serial, endpoints } => {
+                self.on_link_resolved(serial, endpoints)
+            }
+            RegEvent::ProcCommProbed { pid, comm } => {
+                let previous = self.probed_comm.insert(pid, comm.clone());
+                if previous.as_ref() == Some(&comm) {
+                    Outcome::Suppressed
+                } else {
+                    Outcome::Applied
+                }
+            }
+            RegEvent::Removed { id } => self.on_removed(id),
+            RegEvent::ServerSynced => {
+                let already = self.server_synced;
+                self.server_synced = true;
+                self.maybe_complete();
+                if already {
+                    Outcome::Suppressed
+                } else {
+                    Outcome::Applied
+                }
+            }
+            RegEvent::Tick { now } => {
+                self.last_now = now;
+                self.maybe_timeout(now);
+                Outcome::Applied
+            }
+        }
+    }
+
+    /// First resolution *and* every later property change (v3.5 §6.7
+    /// decision 2). The model distinguishes them by what it already holds, so
+    /// the adapter can forward every `info` callback unconditionally.
+    fn on_node_info(&mut self, serial: Serial, observation: NodeObservation) -> Outcome {
+        let Some(entry) = self.nodes.get_mut(&serial) else {
+            // A late `info` for a node already removed. Re-inserting it here
+            // would resurrect a dead node with no id index behind it.
+            tracing::debug!(serial = serial.0, "observer: node info for an unknown node");
+            return Outcome::Suppressed;
+        };
+        if entry.obs.as_ref() == Some(&observation) {
+            // The state-only `info` callbacks PipeWire emits constantly: same
+            // properties, so the projection is provably identical.
+            return Outcome::Suppressed;
+        }
+        entry.obs = Some(observation);
+        // The first `info` retires this node's obligation, which can be the
+        // last one outstanding.
+        self.maybe_complete();
+        Outcome::Applied
+    }
+
+    fn on_device_info(&mut self, serial: Serial, props: DeviceProps) -> Outcome {
+        let Some(entry) = self.devices.get_mut(&serial) else {
+            tracing::debug!(
+                serial = serial.0,
+                "observer: device info for an unknown device"
+            );
+            return Outcome::Suppressed;
+        };
+        if entry.props.as_ref() == Some(&props) {
+            return Outcome::Suppressed;
+        }
+        entry.props = Some(props);
+        // Resolving a Device admits every node that was withheld on it —
+        // which happens at projection time; here it can only retire
+        // obligations.
+        self.maybe_complete();
+        Outcome::Applied
+    }
+
+    fn on_link_added(&mut self, serial: Serial, id: GlobalId, endpoints: Option<LinkEndpoints>) {
+        self.push_id(id, Slot::Link(serial));
+        match endpoints {
+            Some(e) => {
+                self.links.insert(serial, link_snapshot(serial, id, e));
+            }
+            None => {
+                // Correctness path: withhold the Link until the bind fallback
+                // resolves it. Counts as an outstanding obligation.
+                self.pending_links.insert(serial, id);
+            }
+        }
+        self.maybe_complete();
+    }
+
+    fn on_link_resolved(&mut self, serial: Serial, endpoints: LinkEndpoints) -> Outcome {
+        // `remove` also guards against a stale resolution for a Link already
+        // gone: unknown serial ⇒ ignore.
+        if let Some(id) = self.pending_links.remove(&serial) {
+            self.links
+                .insert(serial, link_snapshot(serial, id, endpoints));
+            self.maybe_complete();
+            Outcome::Applied
+        } else {
+            Outcome::Suppressed
+        }
+    }
+
+    fn on_removed(&mut self, id: GlobalId) -> Outcome {
+        let Some(queue) = self.live_ids.get_mut(&id) else {
+            tracing::warn!(global_id = id.0, "observer: remove for an id we never saw");
+            return Outcome::Suppressed;
+        };
+        // Oldest generation first — the id may be shared during a
+        // missed-removal window.
+        let slot = queue.pop_front();
+        if queue.is_empty() {
+            self.live_ids.remove(&id);
+        }
+        match slot {
+            Some(Slot::Node(serial)) => {
+                self.nodes.remove(&serial);
+            }
+            Some(Slot::Port(serial)) => {
+                self.ports.remove(&serial);
+            }
+            Some(Slot::Link(serial)) => {
+                self.links.remove(&serial);
+                self.pending_links.remove(&serial);
+            }
+            Some(Slot::Client(serial)) => {
+                self.clients.remove(&serial);
+            }
+            Some(Slot::Device(serial)) => {
+                self.devices.remove(&serial);
+            }
+            None => {
+                tracing::warn!(global_id = id.0, "observer: empty id slot on remove");
+                return Outcome::Suppressed;
+            }
+        }
+        // A removal can drain the last obligation (an unbound node, a node
+        // withheld on a Device, or a pending link vanished before it
+        // resolved).
+        self.maybe_complete();
+        Outcome::Applied
+    }
+
+    fn push_id(&mut self, id: GlobalId, slot: Slot) {
+        self.live_ids.entry(id).or_default().push_back(slot);
+    }
+
+    /// The bound properties of the Device a node claims by global id, or
+    /// `None` when that claim is unresolved — which covers every fail-closed
+    /// case at once: no such Device observed, its bind still outstanding, or
+    /// **the id claimed by more than one live global**, where there is no way
+    /// to tell whose properties these are (v3.4 §6.1.3).
+    ///
+    /// ⚠️ The ambiguity test is "**exactly one** live global holds this id",
+    /// not "exactly one live *Device*" (Codex phase-3r review, finding 2).
+    /// The weaker test looks equivalent and is not: with `[Device, Port]` on
+    /// one id — a missed removal, the same precondition as every other
+    /// recycled-id hazard — it keeps answering with the older Device's
+    /// properties, so a node claiming that id holds a stale
+    /// `session_device = true`. That flag *removes* the node's owner keys and
+    /// its fail-closed backstop, so a forwarder wearing it can put its output
+    /// leg back on the eligible side: echo, from a lookup that was merely
+    /// looking at the wrong object type.
+    fn device_props(&self, id: GlobalId) -> Option<&DeviceProps> {
+        let slots = self.live_ids.get(&id)?;
+        if slots.len() != 1 {
+            return None; // Ambiguous ⇒ unresolved ⇒ withheld.
+        }
+        let Slot::Device(serial) = slots.front()? else {
+            // The id is live, but it is not a Device any more.
+            return None;
+        };
+        self.devices.get(serial)?.props.as_ref()
+    }
+
+    /// Classify one node's device claim against the currently resolved
+    /// Devices. Recomputed per projection rather than cached at admission:
+    /// the inputs (this node's props, its Device's props) both change over an
+    /// object's lifetime now, and a cached classification is exactly the kind
+    /// of stale provisional answer §6.1.3 forbids.
+    fn classification(&self, obs: &NodeObservation) -> Classification {
+        let device = obs
+            .device_claim
+            .device_id
+            .and_then(|id| self.device_props(id));
+        classify::classify(&obs.device_claim, device)
+    }
+
+    /// Every obligation that must clear before the initial graph is trusted:
+    /// no node awaiting its bind, no node withheld on an unresolved Device,
+    /// no Link awaiting its bind.
+    fn obligations_outstanding(&self) -> bool {
+        if !self.pending_links.is_empty() {
+            return true;
+        }
+        self.nodes.values().any(|entry| match &entry.obs {
+            None => true,
+            Some(obs) => matches!(self.classification(obs), Classification::Withhold { .. }),
+        })
+    }
+
+    /// Completion needs no clock — only the sync flag and an empty obligation
+    /// set — so it may fire on any mutating event. Sticky once reached.
+    fn maybe_complete(&mut self) {
+        if self.readiness != Readiness::Waiting {
+            return;
+        }
+        if self.server_synced && !self.obligations_outstanding() {
+            self.readiness = Readiness::Complete;
+            tracing::info!("observer: readiness epoch reached (synced + no obligations)");
+        }
+    }
+
+    /// Only the timeout consults the clock.
+    fn maybe_timeout(&mut self, now: Millis) {
+        if self.readiness != Readiness::Waiting {
+            return;
+        }
+        if now >= self.deadline {
+            self.readiness = Readiness::TimedOut;
+            tracing::warn!(
+                unbound_nodes = self.unbound_node_count(),
+                withheld = self.withheld_node_count(),
+                pending_links = self.pending_links.len(),
+                "observer: readiness epoch timed out with obligations outstanding — fail closed"
+            );
+        }
+    }
+
+    /// Nodes whose bind has not delivered `info` yet — diagnostics only.
+    fn unbound_node_count(&self) -> usize {
+        self.nodes
+            .values()
+            .filter(|entry| entry.obs.is_none())
+            .count()
+    }
+
+    /// Nodes held out on an unresolved Device — diagnostics only.
+    fn withheld_node_count(&self) -> usize {
+        self.nodes
+            .values()
+            .filter(|entry| {
+                entry.obs.as_ref().is_some_and(|obs| {
+                    matches!(self.classification(obs), Classification::Withhold { .. })
+                })
+            })
+            .count()
+    }
+
+    /// pipewire-pulse's PID from the current clients, validated against the
+    /// probed `comm`. `None` whenever anything is ambiguous or unconfirmed —
+    /// the safe answer (key 4 unusable).
+    fn pulse_pid(&self) -> Option<u32> {
+        let candidate = self.pulse_pid_candidate()?;
+        let comm = self.probed_comm.get(&candidate).and_then(|c| c.as_deref());
+        pulse_pid::validate(candidate, comm)
+    }
+
+    /// Project the current state into the taint engine's inputs.
+    ///
+    /// A node enters the snapshot only if its bind has delivered `info`
+    /// **and** its device claim classifies terminally; anything else is
+    /// withheld (and is already holding `graph_ready` false).
+    pub fn project(&self) -> Projection {
+        let nodes: Vec<NodeSnapshot> = self
+            .nodes
+            .iter()
+            .filter_map(|(&serial, entry)| {
+                let obs = entry.obs.as_ref()?;
+                let session_device = match self.classification(obs) {
+                    Classification::Withhold { .. } => return None,
+                    Classification::SessionDevice => true,
+                    Classification::NotADevice | Classification::NotSessionDevice => false,
+                };
+                let mut props = obs.props.clone();
+                props.session_device = session_device;
+                Some(NodeSnapshot {
+                    serial,
+                    id: entry.id,
+                    name: obs.name.clone(),
+                    role: obs.role,
+                    props,
+                })
+            })
+            .collect();
+        let snapshot = GraphSnapshot::new(
+            nodes,
+            self.ports.values().cloned().collect(),
+            self.links.values().cloned().collect(),
+            self.clients.values().cloned().collect(),
+        );
+        Projection {
+            snapshot,
+            pipewire_pulse_pid: self.pulse_pid(),
+            graph_ready: self.graph_ready(),
+            readiness: self.readiness,
+        }
+    }
+}
+
+fn link_snapshot(serial: Serial, id: GlobalId, e: LinkEndpoints) -> LinkSnapshot {
+    LinkSnapshot {
+        serial,
+        id,
+        output_node: e.output_node,
+        input_node: e.input_node,
+        output_port: e.output_port,
+        input_port: e.input_port,
+    }
+}

src/host/observer/pulse_pid.rs

@@ -0,0 +1,89 @@
+//! Deriving pipewire-pulse's own PID — pure, no PipeWire and no `/proc` I/O.
+//!
+//! The owner bridge's key 4 is `application.process.id`. For a stream created
+//! by a **Pulse-emulated** client that PID is *pipewire-pulse's own*, shared
+//! verbatim across every unrelated Pulse app, so bridging on it would fuse
+//! every Pulse module into one tainted owner (design v3.4 §5.2 correction 5,
+//! §6.1.2). The engine therefore needs to know that one PID so it can refuse
+//! to bridge on it — and **every** way of deriving it can fail, in which case
+//! the safe answer is `None`: key 4 becomes unusable (coarser, never wrong).
+//!
+//! The derivation is split into two pure stages so the I/O — reading
+//! `/proc/<pid>/comm` — stays in the adapter:
+//!
+//! 1. [`candidate`] finds the PID that *looks* like pulse from the graph
+//!    alone: the `pipewire.sec.pid` value shared across multiple Clients.
+//!    Native PipeWire clients carry their own distinct PID; only the
+//!    Pulse shim repeats one value, so a repeated value is the signal.
+//! 2. [`validate`] confirms that candidate against the `comm` the adapter
+//!    read from `/proc`. This is what closes **PID reuse**: a recycled PID
+//!    that coincidentally repeats in the graph is rejected because
+//!    `/proc/<pid>/comm` now names a different process.
+//!
+//! Any failure at either stage — no repeated value, two repeated values,
+//! the property missing, `/proc` gone, a `comm` mismatch — yields `None`.
+
+use crate::host::taint::snapshot::ClientSnapshot;
+use std::collections::BTreeMap;
+
+/// The kernel `comm` of the pipewire-pulse process. `comm` is truncated to
+/// 15 bytes by the kernel; `pipewire-pulse` is 14 bytes, so it is exact —
+/// and exact is the only safe match, since a prefix match would accept a
+/// recycled PID belonging to e.g. `pipewire-pulseX`.
+const PULSE_COMM: &str = "pipewire-pulse";
+
+/// Stage 1: the PID that looks like pipewire-pulse from the client graph.
+///
+/// Returns `Some(pid)` only when **exactly one** `pipewire.sec.pid` value is
+/// shared by two or more clients. Rationale, matched to the failure matrix:
+///
+/// - **consistent** — one value repeats, the rest (native clients) are
+///   distinct ⇒ that value.
+/// - **inconsistent** — two or more values each repeat ⇒ we cannot tell which
+///   is pulse ⇒ `None`.
+/// - **missing property** — the Pulse clients carry no `sec_pid` ⇒ nothing
+///   repeats ⇒ `None`.
+///
+/// A count threshold of two is deliberate: a single client carrying a PID is
+/// indistinguishable from a lone native app, and pulse always mints many.
+pub fn candidate(clients: &[ClientSnapshot]) -> Option<u32> {
+    let mut counts: BTreeMap<u32, usize> = BTreeMap::new();
+    for client in clients {
+        if let Some(pid) = client.sec_pid {
+            *counts.entry(pid).or_insert(0) += 1;
+        }
+    }
+
+    // Every PID seen on 2+ clients is a pulse candidate. If there is exactly
+    // one such PID we trust it; zero or several ⇒ fail closed.
+    let mut repeated = counts.iter().filter(|&(_, &n)| n >= 2).map(|(&pid, _)| pid);
+    let first = repeated.next()?;
+    if repeated.next().is_some() {
+        // Ambiguous: more than one value repeats.
+        return None;
+    }
+    Some(first)
+}
+
+/// Stage 2: confirm the candidate against the `comm` read from
+/// `/proc/<candidate>/comm`.
+///
+/// `comm` is `None` when the adapter's read failed — the `/proc` entry is
+/// gone (the process exited between derivation and probe) — which is itself a
+/// reason to fail closed. A present-but-different `comm` is the **PID reuse**
+/// guard: the number is live but now belongs to someone else.
+pub fn validate(candidate: u32, comm: Option<&str>) -> Option<u32> {
+    match comm {
+        Some(PULSE_COMM) => Some(candidate),
+        _ => None,
+    }
+}
+
+/// The two stages composed, for callers that already hold the probed `comm`.
+/// The model keeps them separate (it recomputes the candidate as clients
+/// churn, and only re-probes when the candidate *changes*), so this is a
+/// convenience for tests and for the fully-resolved path.
+pub fn derive(clients: &[ClientSnapshot], comm_of: impl Fn(u32) -> Option<String>) -> Option<u32> {
+    let candidate = candidate(clients)?;
+    validate(candidate, comm_of(candidate).as_deref())
+}

src/host/pipeline.rs

@@ -189,9 +189,32 @@ fn build_args(
         "!".into(),
         "queue".into(),
         "!".into(),
-        "fdsink".into(),
-        "fd=1".into(),
     ];
+    // Debug A/V-drift tap: when PIXELPASS_TS_DUMP=<path> is set, tee the exact
+    // muxed TS both to fd=1 (normal serve path, unchanged) and to a file, so the
+    // host-side stream can be ffprobe'd for capture-side audio/video PTS drift.
+    // Each tee branch has its own queue so the disk sink can't backpressure the
+    // live serve branch. No effect when unset. (Mirrors PIXELPASS_GST_DEBUG.)
+    if let Some(dump) = std::env::var_os("PIXELPASS_TS_DUMP") {
+        let path = dump.to_string_lossy().into_owned();
+        args.extend([
+            "tee".into(),
+            "name=dbgtee".into(),
+            "!".into(),
+            "queue".into(),
+            "!".into(),
+            "fdsink".into(),
+            "fd=1".into(),
+            "dbgtee.".into(),
+            "!".into(),
+            "queue".into(),
+            "!".into(),
+            "filesink".into(),
+            format!("location={path}"),
+        ]);
+    } else {
+        args.extend(["fdsink".into(), "fd=1".into()]);
+    }
 
     // Downscale step for the quality presets. `None` = encode at native size
     // (the "Source" preset, or a source already at/below the target height — we

src/host/taint/fixture.rs

@@ -0,0 +1,403 @@
+//! Synthetic graph builders for the taint-engine tests.
+//!
+//! Serials are handed out monotonically and never reused, exactly as
+//! PipeWire does; global ids are handed out separately and **may be reused
+//! on purpose**, which is what the recycling tests need.
+
+use std::collections::BTreeMap;
+
+use super::snapshot::{
+    ClientSnapshot, GlobalId, GraphSnapshot, LinkSnapshot, MediaRole, NodeProps, NodeSnapshot,
+    PortDirection, PortSnapshot, Serial,
+};
+
+/// pipewire-pulse's PID, as measured on the target machine.
+pub const PULSE_PID: u32 = 2541;
+/// WirePlumber's PID — one process owning every device node on the box.
+pub const SESSION_PID: u32 = 900;
+
+/// A node's identity in a fixture: what tests pass around.
+#[derive(Clone, Copy, PartialEq, Eq, Debug)]
+pub struct NodeRef {
+    pub serial: Serial,
+    pub id: GlobalId,
+}
+
+#[derive(Default)]
+pub struct Graph {
+    next_serial: u64,
+    next_id: u32,
+    nodes: Vec<NodeSnapshot>,
+    ports: Vec<PortSnapshot>,
+    links: Vec<LinkSnapshot>,
+    clients: Vec<ClientSnapshot>,
+    /// One client connection per process / per module, which is what the
+    /// live graph looks like. Tests that need the *split*-client shape
+    /// (GStreamer opens one per stream) pass clients explicitly instead.
+    client_by_app: BTreeMap<u32, GlobalId>,
+    client_by_module: BTreeMap<u64, GlobalId>,
+    /// Native (non-Pulse-emulated) clients, whose `pipewire.sec.pid` is the
+    /// app's **own** pid rather than pipewire-pulse's. See
+    /// [`Graph::native_client_node`].
+    native_client_by_app: BTreeMap<u32, GlobalId>,
+    session_client: Option<GlobalId>,
+}
+
+impl Graph {
+    pub fn new() -> Self {
+        Self {
+            // Start past u32::MAX so every fixture also exercises the phase
+            // 0a widening: a serial that a u32 model would have truncated.
+            next_serial: u64::from(u32::MAX) + 1,
+            next_id: 1,
+            ..Self::default()
+        }
+    }
+
+    fn serial(&mut self) -> Serial {
+        self.next_serial += 1;
+        Serial(self.next_serial)
+    }
+
+    fn id(&mut self) -> GlobalId {
+        self.next_id += 1;
+        GlobalId(self.next_id)
+    }
+
+    /// A client object. `sec_pid` is `pipewire.sec.pid` — pipewire-pulse's
+    /// PID for Pulse-emulated clients.
+    pub fn client(&mut self, sec_pid: Option<u32>) -> GlobalId {
+        let serial = self.serial();
+        let id = self.id();
+        self.clients.push(ClientSnapshot {
+            serial,
+            id,
+            sec_pid,
+        });
+        id
+    }
+
+    /// The client connection an ordinary process holds — one per PID,
+    /// created on demand.
+    pub fn client_of_app(&mut self, pid: u32) -> GlobalId {
+        if let Some(id) = self.client_by_app.get(&pid) {
+            return *id;
+        }
+        let id = self.client(Some(PULSE_PID));
+        self.client_by_app.insert(pid, id);
+        id
+    }
+
+    /// A **native PipeWire** client's stream: `client.id` on the node, **no
+    /// `application.process.id`**, and the app's real pid only on the Client
+    /// as `pipewire.sec.pid`.
+    ///
+    /// ⚠️ This is what an ordinary app actually looks like when it does not go
+    /// through pipewire-pulse — measured for mpv on its default ao and for
+    /// peerspeak's own playback stream. [`Graph::app_node`] models the
+    /// Pulse-emulated shape, where the pid is on the node and the Client's
+    /// `sec_pid` is the *daemon's*; both shapes are live on this host, and
+    /// only this one exercises key 4's Client fallback (round 10, R10-3).
+    pub fn native_client_node(&mut self, name: &str, role: MediaRole, pid: u32) -> NodeRef {
+        let client = match self.native_client_by_app.get(&pid) {
+            Some(id) => *id,
+            None => {
+                let id = self.client(Some(pid));
+                self.native_client_by_app.insert(pid, id);
+                id
+            }
+        };
+        self.node(
+            name,
+            role,
+            NodeProps {
+                client_id: Some(client),
+                ..NodeProps::default()
+            },
+        )
+    }
+
+    /// An ordinary application stream: its own client, its own PID.
+    pub fn app_node(&mut self, name: &str, role: MediaRole, pid: u32) -> NodeRef {
+        let client = self.client_of_app(pid);
+        self.node(name, role, app(client, pid))
+    }
+
+    /// The client a pactl module holds. Measured: each module gets its own
+    /// (`sink-sunshine-*` were clients 83/86/92), which is why one tainted
+    /// module does not fuse with the next.
+    pub fn client_of_module(&mut self, module: u64) -> GlobalId {
+        match self.client_by_module.get(&module) {
+            Some(id) => *id,
+            None => {
+                let id = self.client(Some(PULSE_PID));
+                self.client_by_module.insert(module, id);
+                id
+            }
+        }
+    }
+
+    /// A leg of a pactl-loaded module: one client per module, and the
+    /// node's `application.process.id` is **pipewire-pulse's own**, because
+    /// pipewire-pulse genuinely is the client.
+    pub fn module_node(&mut self, name: &str, role: MediaRole, module: u64) -> NodeRef {
+        let client = self.client_of_module(module);
+        self.node(name, role, pulse_module(client, module, PULSE_PID))
+    }
+
+    /// A leg joined to its siblings by `node.link-group` — loopback,
+    /// filter-chain, echo-cancel.
+    pub fn group_node(&mut self, name: &str, role: MediaRole, group: &str, pid: u32) -> NodeRef {
+        let client = self.client_of_app(pid);
+        self.node(name, role, link_group(group, client, pid))
+    }
+
+    /// A device node as the session manager creates it: no strong key,
+    /// WirePlumber's client and PID — shared with every other device — and
+    /// a `device.id`, which is what marks it as session-manager-exported.
+    pub fn device_node(&mut self, name: &str, role: MediaRole) -> NodeRef {
+        let session = match self.session_client {
+            Some(id) => id,
+            None => {
+                let id = self.client(None);
+                self.session_client = Some(id);
+                id
+            }
+        };
+        self.node(name, role, device(session, SESSION_PID))
+    }
+
+    /// A node that *belongs to* a Device but is not a passive device node —
+    /// a filter associated with a card. Phase 3 must not classify this as a
+    /// session device, or it loses both its coarse owner keys and its
+    /// ability to trip the fail-closed backstop.
+    pub fn device_associated_filter(&mut self, name: &str, role: MediaRole, pid: u32) -> NodeRef {
+        let client = self.client_of_app(pid);
+        self.node(name, role, app(client, pid))
+    }
+
+    /// A **virtual** sink an application created natively: an `Audio/Sink`
+    /// with no `device.id` and no strong key, sharing one client with the
+    /// stream that re-emits what it receives. Coarse keys must still bridge
+    /// these two, or the whole call leaks through the re-emitting leg.
+    pub fn native_virtual_node(&mut self, name: &str, role: MediaRole, pid: u32) -> NodeRef {
+        let client = self.client_of_app(pid);
+        self.node(name, role, app(client, pid))
+    }
+
+    /// A peerspeak-owned node carrying **both** ownership carriers, as a
+    /// live one does. `name` gets the real `node.name` prefix so the fixture
+    /// cannot pass on the property alone.
+    pub fn peerspeak_node(&mut self, name: &str, pid: u32) -> NodeRef {
+        let client = self.client_of_app(pid);
+        let name = format!("{}{name}_{pid}", super::PEERSPEAK_OWNED_NODE_PREFIX);
+        self.node(&name, MediaRole::StreamOutput, peerspeak_owned(client, pid))
+    }
+
+    /// Both ownership carriers on a node of **any** role — an impostor, or a
+    /// producer-side tagging bug. Only [`MediaRole::StreamOutput`] makes it a
+    /// taint root (round 10, R10-1); every other role must be ignored, and
+    /// these are the fixtures that prove it.
+    pub fn peerspeak_tagged_node(&mut self, name: &str, role: MediaRole, pid: u32) -> NodeRef {
+        let client = self.client_of_app(pid);
+        let name = format!("{}{name}_{pid}", super::PEERSPEAK_OWNED_NODE_PREFIX);
+        self.node(&name, role, peerspeak_owned(client, pid))
+    }
+
+    /// Carrier 1 alone: the `peerspeak.owned` property present, the
+    /// `node.name` prefix absent. What the engine sees for a node it had to
+    /// bind to observe (v3.5 §6.7).
+    pub fn peerspeak_node_prop_only(&mut self, name: &str, pid: u32) -> NodeRef {
+        let client = self.client_of_app(pid);
+        self.node(name, MediaRole::StreamOutput, peerspeak_owned(client, pid))
+    }
+
+    /// Carrier 2 alone: the `node.name` prefix present, the property absent
+    /// — indistinguishable from an ordinary app in every other respect.
+    /// This is the case that survives the F1 observation defect, and the
+    /// reason round 8 added a second carrier at all.
+    pub fn peerspeak_node_name_only(&mut self, role: &str, pid: u32) -> NodeRef {
+        let client = self.client_of_app(pid);
+        let name = format!("{}{role}_{pid}", super::PEERSPEAK_OWNED_NODE_PREFIX);
+        self.node(&name, MediaRole::StreamOutput, app(client, pid))
+    }
+
+    pub fn node(&mut self, name: &str, role: MediaRole, props: NodeProps) -> NodeRef {
+        let id = self.id();
+        self.node_with_id(name, role, id, props)
+    }
+
+    /// Force a global id — for reproducing id recycling after teardown.
+    pub fn node_with_id(
+        &mut self,
+        name: &str,
+        role: MediaRole,
+        id: GlobalId,
+        props: NodeProps,
+    ) -> NodeRef {
+        let serial = self.serial();
+        self.nodes.push(NodeSnapshot {
+            serial,
+            id,
+            name: Some(name.to_string()),
+            role,
+            props,
+        });
+        NodeRef { serial, id }
+    }
+
+    pub fn port(&mut self, node: NodeRef, direction: PortDirection, exclusive: bool) {
+        let serial = self.serial();
+        let id = self.id();
+        self.ports.push(PortSnapshot {
+            serial,
+            id,
+            node: node.id,
+            direction,
+            exclusive,
+            monitor: false,
+        });
+    }
+
+    /// A signal edge: audio flows `from → to`.
+    pub fn link(&mut self, from: NodeRef, to: NodeRef) {
+        self.link_ids(from.id, to.id);
+    }
+
+    /// A link naming raw ids, so a test can dangle an endpoint.
+    pub fn link_ids(&mut self, from: GlobalId, to: GlobalId) {
+        let serial = self.serial();
+        let id = self.id();
+        self.links.push(LinkSnapshot {
+            serial,
+            id,
+            output_node: from,
+            input_node: to,
+            output_port: None,
+            input_port: None,
+        });
+    }
+
+    /// An id that belongs to nothing — for unresolved-endpoint tests.
+    pub fn dangling_id(&mut self) -> GlobalId {
+        self.id()
+    }
+
+    pub fn build(&self) -> GraphSnapshot {
+        self.build_without(&[])
+    }
+
+    /// A later snapshot in which some nodes have gone away, along with
+    /// their ports and every link touching them. Surviving objects keep
+    /// their serials, which is what makes sticky-taint sequences testable.
+    pub fn build_without(&self, dropped: &[NodeRef]) -> GraphSnapshot {
+        let gone_serials: Vec<Serial> = dropped.iter().map(|n| n.serial).collect();
+        let nodes: Vec<NodeSnapshot> = self
+            .nodes
+            .iter()
+            .filter(|n| !gone_serials.contains(&n.serial))
+            .cloned()
+            .collect();
+        // Filter by what was *dropped*, not by what is live: a link to an id
+        // that never had a node is a dangling endpoint, and dropping those
+        // here would quietly disarm every unresolved-ancestry test.
+        let gone_ids: Vec<GlobalId> = dropped.iter().map(|n| n.id).collect();
+        GraphSnapshot::new(
+            nodes,
+            self.ports
+                .iter()
+                .filter(|p| !gone_ids.contains(&p.node))
+                .cloned()
+                .collect(),
+            self.links
+                .iter()
+                .filter(|l| !gone_ids.contains(&l.output_node) && !gone_ids.contains(&l.input_node))
+                .cloned()
+                .collect(),
+            self.clients.clone(),
+        )
+    }
+
+    /// Drop clients too — full owner teardown.
+    ///
+    /// Invalidates the per-app/per-module caches as well: leaving them
+    /// stale made a later `client_of_app` hand back the *removed* client's
+    /// id, so a test that meant "a brand-new client after teardown" was
+    /// really building a node pointing at a client object that no longer
+    /// existed (Codex round 1, finding 8).
+    pub fn drop_clients(&mut self, ids: &[GlobalId]) {
+        self.clients.retain(|c| !ids.contains(&c.id));
+        self.client_by_app.retain(|_, id| !ids.contains(id));
+        self.client_by_module.retain(|_, id| !ids.contains(id));
+        if self.session_client.is_some_and(|id| ids.contains(&id)) {
+            self.session_client = None;
+        }
+    }
+
+    /// A client that reuses a global id a dead client had — the recycling
+    /// case, with a fresh serial.
+    pub fn client_with_id(&mut self, id: GlobalId, sec_pid: Option<u32>) -> GlobalId {
+        let serial = self.serial();
+        self.clients.push(ClientSnapshot {
+            serial,
+            id,
+            sec_pid,
+        });
+        id
+    }
+}
+
+/// An ordinary application stream: real PID, one client connection.
+pub fn app(client: GlobalId, pid: u32) -> NodeProps {
+    NodeProps {
+        client_id: Some(client),
+        process_id: Some(pid),
+        ..NodeProps::default()
+    }
+}
+
+/// A pactl-module-created stream: the daemon is the client, so the node's
+/// `application.process.id` is pipewire-pulse's own.
+pub fn pulse_module(client: GlobalId, module: u64, pulse_pid: u32) -> NodeProps {
+    NodeProps {
+        pulse_module_id: Some(module),
+        client_id: Some(client),
+        process_id: Some(pulse_pid),
+        ..NodeProps::default()
+    }
+}
+
+/// A PipeWire-module leg joined to its siblings by `node.link-group`
+/// (loopback, filter-chain, echo-cancel).
+pub fn link_group(group: &str, client: GlobalId, pid: u32) -> NodeProps {
+    NodeProps {
+        link_group: Some(group.to_string()),
+        client_id: Some(client),
+        process_id: Some(pid),
+        ..NodeProps::default()
+    }
+}
+
+/// A device node as the session manager creates it: no strong key, and the
+/// session manager's own client and PID — shared with every other device.
+///
+/// Measured 2026-07-21: real ALSA device nodes carry the shared
+/// `client.id` but **no** `application.process.id` at all. Giving them one
+/// here is deliberately *more* pessimistic than reality — it hands the
+/// engine a second coarse key it could fuse devices on, so a test that
+/// passes here also passes against the real props.
+pub fn device(session_client: GlobalId, session_pid: u32) -> NodeProps {
+    NodeProps {
+        client_id: Some(session_client),
+        process_id: Some(session_pid),
+        session_device: true,
+        ..NodeProps::default()
+    }
+}
+
+pub fn peerspeak_owned(client: GlobalId, pid: u32) -> NodeProps {
+    NodeProps {
+        peerspeak_owned: true,
+        ..app(client, pid)
+    }
+}

src/host/taint/owner.rs

@@ -0,0 +1,561 @@
+//! The owner bridge — grouping nodes that belong to the same *owner* even
+//! though the graph shows no Link between them.
+//!
+//! This is the subtlest part of the design (v3.4 §6.1.2). Measured fact it
+//! exists to handle: a `module-loopback` forwarder's input leg and output
+//! leg have **no Link between them**, so walking Links alone from the
+//! leaking output leg finds no inbound links at all — a dead end that reads
+//! as "clean". The legs are related only by shared properties.
+//!
+//! ## The rule
+//!
+//! A union of keys, strongest first:
+//!
+//! | # | key | scope |
+//! | --- | --- | --- |
+//! | 1 | `node.link-group` | per module/filter instance |
+//! | 2 | `pulse.module.id` | per pactl module |
+//! | 3 | `client.id` | per **connection** |
+//! | 4 | `application.process.id` | per process |
+//!
+//! ⚠️ **"Resolves" means the two legs carry the key AND the values are
+//! EQUAL — not "the first key present".** A first-present implementation
+//! reproduces the exact measured leak: for `gst-launch pulsesrc ! pulsesink`
+//! both legs carry `client.id` (209 and 210) but the values *differ*, so
+//! first-present stops at key 3, sees a mismatch, and concludes "different
+//! owners". The legs are in fact one process (`application.process.id`
+//! 20172 on both). So: try each key in order, and a key resolves only if
+//! both legs carry it and the values are equal; otherwise fall through.
+//!
+//! ## Two exceptions, both guarding against mass over-exclusion
+//!
+//! 1. **Never bridge on key 4 when the value is pipewire-pulse's own PID**
+//!    (v3.4 §6.1.2). Module-created streams all carry the daemon's PID, so
+//!    bridging on it fuses every Pulse module into one owner and a single
+//!    tainted module input would exclude every module-created stream on the
+//!    box. Keys 1 and 2 already cover those cases precisely.
+//!
+//! 2. **Coarse keys (3 and 4) may not bridge nodes exported from a real
+//!    `Device`** — i.e. nodes carrying `device.id`. ⚠️ This rule is *not*
+//!    in design v3.4; it was found while implementing, and it is the exact
+//!    analogue of exception 1 for the session manager.
+//!    ✅ **MEASURED on the live graph 2026-07-21:**
+//!
+//!    | node | `client.id` | `device.id` | `factory.name` |
+//!    | --- | --- | --- | --- |
+//!    | 5 × `alsa_{output,input}.*` | **42** (`WirePlumber [export]`) | 43/45/46 | `api.alsa.pcm.{sink,source}` |
+//!    | 3 × `sink-sunshine-*` | 83 / 86 / 92 (each its own) | **absent** | `support.null-audio-sink` |
+//!
+//!    So one shared coarse key genuinely does relate every hardware device
+//!    on the box, and `device.id` cleanly separates that set from virtual
+//!    sinks. Without the rule, the hardware sink carrying peerspeak's
+//!    playback (tainted by design, every single recompute) would bridge to
+//!    *every other device node including the microphone source*, whose
+//!    readers would then taint their owners' playback legs — reproducing
+//!    precisely the §6.1.1 catastrophe ("excludes any app using a
+//!    microphone") through a different door.
+//!
+//!    ⚠️ **Keyed on `device.id`, NOT on `media.class` being `Audio/Sink`.**
+//!    The first cut suppressed coarse keys for every device-*role* node,
+//!    and Codex refuted it: a **native virtual sink** — an app that creates
+//!    an `Audio/Sink` plus a re-emitting stream on one client, with no
+//!    `link-group` and no `pulse.module.id` — would then have had its only
+//!    correlation stripped, and it would have leaked the whole call. Such a
+//!    sink has no `device.id`, so it now bridges on `client.id` as it
+//!    should.
+//!
+//! Grouping is **transitive** (union-find). That is the fail-closed
+//! direction: bigger owner components mean more taint, never less.
+
+use std::collections::{BTreeMap, BTreeSet};
+
+use super::snapshot::{GlobalId, GraphSnapshot, NodeSnapshot, Serial};
+
+/// Everything owner-key derivation needs from outside a single node.
+///
+/// Introduced by round 10 (R10-3). Before it, `keys_of` read only node
+/// properties, and key 4 was therefore available **only** to nodes carrying
+/// `application.process.id` — which native PipeWire clients do not. mpv on its
+/// default ao, and peerspeak's own playback stream, expose nothing but
+/// `client.id`, so both were *unbounded*, and the moment any tainted reader
+/// existed anywhere, `propagate_unresolved_owner` excluded every one of them.
+/// Measured: an untagged mpv went from eligible (alone) to `unresolved-owner`
+/// the instant peerspeak played audio. That is "native-PipeWire apps are never
+/// shareable", which is not a feature.
+///
+/// The missing pid is not missing at all — it is one hop away, on the node's
+/// **Client**, as `pipewire.sec.pid`, and already in the snapshot.
+pub struct OwnerCtx {
+    pub pipewire_pulse_pid: Option<u32>,
+    /// `client.id` → that Client's `pipewire.sec.pid`.
+    ///
+    /// Clients whose global id is **ambiguous** (two live objects claiming it,
+    /// i.e. the observer missed a removal) are deliberately absent: resolving
+    /// an ambiguous id to a pid would attribute a node to whichever Client won
+    /// a coin toss, and inventing an owner key is the one direction that can
+    /// *reduce* taint. Absent ⇒ unbounded ⇒ fails closed, as before.
+    client_pids: BTreeMap<GlobalId, u32>,
+}
+
+impl OwnerCtx {
+    pub fn new(snapshot: &GraphSnapshot, pipewire_pulse_pid: Option<u32>) -> Self {
+        let mut client_pids: BTreeMap<GlobalId, u32> = BTreeMap::new();
+        // ⚠️ Tracked separately from `client_pids`, and that is the point: a
+        // Client with no `sec_pid` still *claims* its id. Detecting duplicates
+        // by looking in the pid map would let a pid-less first claimant leave
+        // no trace, so the next Client claiming the same id would look unique
+        // and its pid would be used — resolving an ambiguous id, which is the
+        // one guess this guard exists to refuse. Pid-less Clients are ordinary
+        // (the session manager's is one).
+        let mut seen: BTreeSet<GlobalId> = BTreeSet::new();
+        for client in snapshot.clients() {
+            if !seen.insert(client.id) {
+                // Two Clients claiming one id: drop it entirely rather than
+                // pick. See the field docs.
+                client_pids.remove(&client.id);
+                continue;
+            }
+            if let Some(pid) = client.sec_pid {
+                client_pids.insert(client.id, pid);
+            }
+        }
+        Self {
+            pipewire_pulse_pid,
+            client_pids,
+        }
+    }
+
+    /// The `pipewire.sec.pid` of this node's Client, if it has one and that
+    /// Client's id is unambiguous.
+    fn client_pid(&self, node: &NodeSnapshot) -> Option<u32> {
+        self.client_pids.get(&node.props.client_id?).copied()
+    }
+}
+
+/// Which key bridged two legs. Ordered strongest first; the `Ord` derive is
+/// load-bearing for "report the strongest shared key".
+#[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Debug, Hash)]
+pub enum OwnerKey {
+    LinkGroup,
+    PulseModuleId,
+    ClientId,
+    ProcessId,
+}
+
+impl OwnerKey {
+    /// Stable, machine-readable — this ends up in the phase 5 audit output
+    /// and the phase 6 status event.
+    ///
+    /// ⚠️ **Known imprecision, deliberately not fixed here.** `ProcessId` now
+    /// covers two sources — the node's `application.process.id` and its
+    /// Client's `pipewire.sec.pid` (see [`keys_of`]) — so a bridge reported as
+    /// `application.process.id` may in fact have resolved on the Client's
+    /// protected pid. Pre-existing since R10-3 made the Client a fallback, and
+    /// widened by the review's finding 1 making it a union. Splitting it would
+    /// add a code to a set that is explicitly a stable contract for the audit
+    /// output and the "why isn't this app being shared?" answer, so it wants
+    /// its own decision rather than a drive-by.
+    pub fn code(self) -> &'static str {
+        match self {
+            Self::LinkGroup => "node.link-group",
+            Self::PulseModuleId => "pulse.module.id",
+            Self::ClientId => "client.id",
+            Self::ProcessId => "application.process.id",
+        }
+    }
+}
+
+/// The value a node presents for a given key, if it presents one at all.
+#[derive(Clone, PartialEq, Eq, PartialOrd, Ord, Debug)]
+enum KeyValue {
+    Text(String),
+    Num(u64),
+}
+
+/// Owner keys usable on this node, strongest first.
+///
+/// A key that is present but unusable (the pipewire-pulse PID; a coarse key
+/// on a device node) is **absent** here — that is the whole mechanism of the
+/// two exceptions.
+fn keys_of(node: &NodeSnapshot, ctx: &OwnerCtx) -> Vec<(OwnerKey, KeyValue)> {
+    let mut out = Vec::new();
+    if let Some(group) = &node.props.link_group {
+        out.push((OwnerKey::LinkGroup, KeyValue::Text(group.clone())));
+    }
+    if let Some(module) = node.props.pulse_module_id {
+        out.push((OwnerKey::PulseModuleId, KeyValue::Num(module)));
+    }
+    // Exception 2: coarse keys never bridge passive session-manager device
+    // nodes — they all share the session manager's client.
+    if node.props.session_device {
+        return out;
+    }
+    if let Some(client) = node.props.client_id {
+        out.push((OwnerKey::ClientId, KeyValue::Num(u64::from(client.0))));
+    }
+    // Key 4, from the node **and** from its Client (round 10, R10-3; made a
+    // union rather than a fallback by the round-10 review, finding 1).
+    //
+    // ⚠️ **A union, not `node.or_else(client)`, and the difference is a leak.**
+    // The node's `application.process.id` is client-controlled and optional;
+    // the Client's `pipewire.sec.pid` is `pipewire.*`, protected, and the only
+    // one that can carry a soundness argument (the same reason
+    // `propagate_unresolved_owner` sweeps everything for an unbounded reader).
+    // Letting the node's value *replace* the Client's meant one process using
+    // two Clients could escape the bridge entirely: its tainted reader reports
+    // a bogus node pid, its output leg omits the node pid and falls back to
+    // the Client's real one, the two legs are bounded by different values, so
+    // they neither bridge nor trip the unbounded sweep — and the output stays
+    // eligible while re-emitting the call. Carrying both values costs nothing
+    // and closes it: a leg that presents *either* value bridges.
+    //
+    // ⚠️ **Exception 1 applies to each value independently, and that is the
+    // whole risk here.** Measured on this host: 15 unrelated Clients share
+    // `sec_pid` 2528, which is pipewire-pulse's own — every Pulse-emulated app
+    // has one. Suppressing it per value is what keeps the union from fusing
+    // all fifteen into a single owner while still keeping each app's real
+    // per-app pid. For the common Pulse shape (node pid = the app's, Client
+    // `sec_pid` = the daemon's) the union therefore reduces to exactly the
+    // node's pid, as before.
+    //
+    // Note the fail-closed asymmetry when the daemon PID is unknown (`None`):
+    // the exception does *not* fire, key 4 applies to everything, and Pulse
+    // modules fuse into one owner. That is broad over-exclusion — annoying and
+    // safe — which is the direction v3.4 §6.1.2's failure-mode paragraph asks
+    // for.
+    for pid in [node.props.process_id, ctx.client_pid(node)]
+        .into_iter()
+        .flatten()
+    {
+        if Some(pid) == ctx.pipewire_pulse_pid {
+            continue;
+        }
+        let key = (OwnerKey::ProcessId, KeyValue::Num(u64::from(pid)));
+        // The two agree far more often than not; a duplicate entry would be
+        // harmless but would make the audit's key list read oddly.
+        if !out.contains(&key) {
+            out.push(key);
+        }
+    }
+    out
+}
+
+/// Can this node's owner be positively bounded — i.e. can we enumerate its
+/// sibling legs and be right?
+///
+/// ⚠️ Not the same as "has any usable key", and the difference is a leak.
+/// `client.id` alone does **not** bound an owner: that is the measured
+/// GStreamer refutation, where one process presented two different
+/// `client.id`s for its two legs. So an owner is bounded only by a strong
+/// key (link-group / pulse.module.id) or by a *usable* process id — usable
+/// meaning key 4 was not suppressed as pipewire-pulse's own PID.
+///
+/// The case this exists for is v3.4 §12's "module forwarder with neither
+/// `link-group` nor `pulse.module.id`": its process id is the daemon's and
+/// therefore suppressed, its two legs may carry different `client.id`s, and
+/// nothing else relates them. Its sibling output leg cannot be found, so
+/// the engine must fail closed rather than declare it clean
+/// (v3.4 §6.1.1, final paragraph).
+///
+/// # 🔴 OPEN, phase-6 blocking — the key union can *reduce* taint here
+///
+/// **Round 11 review, finding 1. Verified correct; deliberately not fixed in
+/// that round.** Round 10 made key 4 a union of the node's
+/// `application.process.id` and its Client's `pipewire.sec.pid`, and the claim
+/// that this was "strictly additive" was too strong: the same key list also
+/// feeds *this* predicate, so adding a value can move a node from unbounded to
+/// bounded, and `propagate_unresolved_owner`'s global sweep is triggered by an
+/// **un**bounded tainted reader. Concretely:
+///
+/// 1. A tainted reader's node claims the pipewire-pulse PID while its Client
+///    holds a real protected PID `A`. Under `or_else` the node's value won and
+///    exception 1 suppressed it, leaving the reader unbounded; under the union
+///    it is bounded by `A`.
+/// 2. Its process's output leg uses a second Client whose id is **ambiguous**
+///    (the observer missed a removal), so no protected PID is available — but
+///    the leg claims a bogus `application.process.id` `B`, which bounds it.
+/// 3. Neither the bridge nor the sweep fires, and the output stays eligible
+///    while re-emitting the call.
+///
+/// It cannot leak today: `evaluate()` is reached only by the dry-run audit,
+/// which creates no links. It becomes live when phase 6 consumes eligibility.
+///
+/// **Why it is not fixed yet.** The principled repair is provenance: a
+/// self-claimed `application.process.id` is not a *sound* bound, only the
+/// protected keys are. But applying that bluntly makes every Pulse-emulated
+/// app unbounded — their Client's `sec_pid` is the daemon's and suppressed, so
+/// the node's own claim is their only per-app identity — which re-triggers the
+/// §6.1.1 mass over-exclusion the whole design is built to avoid, and would
+/// make the eligible half of the §5.1 matrix empty.
+///
+/// The targeted rule that closes the path above without that cost: **a node
+/// whose Client cannot be resolved at all must not be bounded by its own
+/// self-claimed PID.** An ambiguous Client already means "we do not know who
+/// owns this", and a self-claim must not paper over it; a Pulse app's Client
+/// *is* resolved (to the daemon's PID, then suppressed), so it keeps its
+/// bound. Implementing it needs `OwnerCtx` to distinguish "resolved" from
+/// "absent", and `OwnerKeyIndex` to carry boundedness separately from the key
+/// set, since bridging must keep using the full union.
+///
+/// ⚠️ Do this **with the §5.1 matrix data in hand**, not before: the whole
+/// question is how much over-exclusion the rule actually causes on a real
+/// graph, and that is measurable rather than arguable.
+///
+/// ## Round 12 — the deferral holds, and "resolved" has a trap in it
+///
+/// Codex re-examined this and agreed the deferral is defensible while
+/// `evaluate()` is audit-only, and that the rule above closes the recorded path
+/// without unbounding normal Pulse-emulated apps — **but only under one
+/// reading of "resolves"**, and the wrong reading reintroduces the hole:
+///
+/// - ✅ "Resolved" must mean **an unambiguous Client that yields
+///   `Some(pipewire.sec.pid)`**, taken *before* the pipewire-pulse suppression
+///   step. A Pulse app then still has the daemon's protected PID as
+///   provenance, even though that value is omitted from the bridge keys, so it
+///   stays bounded and the eligible half survives.
+/// - ❌ **Do not** implement it as "a unique Client object exists". A unique
+///   Client with `sec_pid = None` would satisfy that test while providing no
+///   protected identity at all, leaving exactly the self-claimed-PID hole this
+///   rule is meant to close.
+///
+/// So the matrix needs five Client cases, not two: **absent**, **ambiguous**,
+/// **unique but pid-less**, **resolved-native**, and
+/// **resolved-to-pipewire-pulse**. The third is the one that distinguishes the
+/// two readings, and it is the row a two-case matrix would silently skip.
+pub fn owner_is_bounded(node: &NodeSnapshot, ctx: &OwnerCtx) -> bool {
+    keys_of(node, ctx)
+        .iter()
+        .any(|(key, _)| *key != OwnerKey::ClientId)
+}
+
+/// Owner keys computed once per snapshot.
+///
+/// `keys_of` allocates a `Vec` and clones the `link-group` string, and the
+/// bridge asks for keys once per (tainted member × component member) pair —
+/// so recomputing was the hot spot in an otherwise linear pass.
+#[derive(Debug, Default)]
+pub struct OwnerKeyIndex {
+    keys: BTreeMap<Serial, Vec<(OwnerKey, KeyValue)>>,
+}
+
+impl OwnerKeyIndex {
+    pub fn build(snapshot: &GraphSnapshot, ctx: &OwnerCtx) -> Self {
+        Self {
+            keys: snapshot
+                .nodes()
+                .map(|node| (node.serial, keys_of(node, ctx)))
+                .collect(),
+        }
+    }
+
+    /// The strongest key these two nodes share directly, if any.
+    pub fn strongest_shared(&self, a: Serial, b: Serial) -> Option<OwnerKey> {
+        let (Some(a_keys), Some(b_keys)) = (self.keys.get(&a), self.keys.get(&b)) else {
+            return None;
+        };
+        // Stored strongest-first, so the first match is the strongest.
+        a_keys.iter().find_map(|(key, value)| {
+            b_keys
+                .iter()
+                .any(|(other_key, other_value)| other_key == key && other_value == value)
+                .then_some(*key)
+        })
+    }
+
+    /// Is `client.id` a usable owner key for this node?
+    ///
+    /// ⚠️ Load-bearing for sticky state. A device node's `client.id` is
+    /// suppressed by exception 2, so recording the session manager's Client
+    /// as a *member* of a tainted device's sticky owner would smuggle the
+    /// suppressed key back in: the next recompute would expand that Client
+    /// to every hardware node on the box — the microphone included — and
+    /// the §6.1.1 catastrophe would arrive one epoch late instead of never.
+    /// (Codex round 2, finding 1.)
+    pub fn uses_client_key(&self, serial: Serial) -> bool {
+        self.keys
+            .get(&serial)
+            .is_some_and(|keys| keys.iter().any(|(key, _)| *key == OwnerKey::ClientId))
+    }
+
+    /// The owner keys that are safe to remember *across* connections, for
+    /// sticky taint: the strong keys plus a usable process id.
+    ///
+    /// `client.id` is deliberately excluded — it identifies a *connection*,
+    /// and the whole point of a fingerprint is to survive one process
+    /// closing a connection and opening another. A live Client member is
+    /// what covers the same-connection case, precisely.
+    ///
+    /// These are recyclable strings and numbers, so they are only ever
+    /// applied while some **serial** member of the owner is still live
+    /// (v3.4 §6.1.3): while the process is alive, its PID cannot have been
+    /// handed to anyone else.
+    pub fn fingerprints(&self, serial: Serial) -> Vec<Fingerprint> {
+        self.keys
+            .get(&serial)
+            .map(|keys| {
+                keys.iter()
+                    .filter(|(key, _)| *key != OwnerKey::ClientId)
+                    .map(|(key, value)| Fingerprint(*key, value.clone()))
+                    .collect()
+            })
+            .unwrap_or_default()
+    }
+
+    /// Does this node currently present `fingerprint`?
+    pub fn has_fingerprint(&self, serial: Serial, fingerprint: &Fingerprint) -> bool {
+        self.keys.get(&serial).is_some_and(|keys| {
+            keys.iter()
+                .any(|(key, value)| *key == fingerprint.0 && *value == fingerprint.1)
+        })
+    }
+
+    /// See [`owner_is_bounded`].
+    pub fn is_bounded(&self, serial: Serial) -> bool {
+        self.keys
+            .get(&serial)
+            .is_some_and(|keys| keys.iter().any(|(key, _)| *key != OwnerKey::ClientId))
+    }
+}
+
+/// The strongest key two nodes share, or `None` if they share none. Used to
+/// *name* the key in a bridge decision; membership itself is transitive and
+/// comes from [`OwnerComponents`].
+pub fn strongest_shared_key(
+    a: &NodeSnapshot,
+    b: &NodeSnapshot,
+    ctx: &OwnerCtx,
+) -> Option<OwnerKey> {
+    let a_keys = keys_of(a, ctx);
+    let b_keys = keys_of(b, ctx);
+    // `keys_of` yields strongest-first, so the first match is the strongest.
+    a_keys.iter().find_map(|(key, value)| {
+        b_keys
+            .iter()
+            .any(|(other_key, other_value)| other_key == key && other_value == value)
+            .then_some(*key)
+    })
+}
+
+/// A remembered owner key — see [`OwnerKeyIndex::fingerprints`].
+#[derive(Clone, PartialEq, Eq, PartialOrd, Ord, Debug)]
+pub struct Fingerprint(OwnerKey, KeyValue);
+
+/// Nodes partitioned into owner components.
+#[derive(Clone, Debug, Default)]
+pub struct OwnerComponents {
+    /// node serial → component index.
+    of_node: BTreeMap<Serial, usize>,
+    /// component index → member node serials, ascending.
+    members: Vec<Vec<Serial>>,
+}
+
+impl OwnerComponents {
+    pub fn build(snapshot: &GraphSnapshot, ctx: &OwnerCtx) -> Self {
+        let serials: Vec<Serial> = snapshot.nodes().map(|n| n.serial).collect();
+        let index: BTreeMap<Serial, usize> =
+            serials.iter().enumerate().map(|(i, s)| (*s, i)).collect();
+        let mut uf = UnionFind::new(serials.len());
+
+        // Group by (key, value) and union within each group. Equivalent to
+        // the pairwise "some key resolves" rule, and O(n log n).
+        let mut buckets: BTreeMap<(OwnerKey, KeyValue), Vec<usize>> = BTreeMap::new();
+        for node in snapshot.nodes() {
+            let slot = index[&node.serial];
+            for (key, value) in keys_of(node, ctx) {
+                buckets.entry((key, value)).or_default().push(slot);
+            }
+        }
+        for group in buckets.values() {
+            for pair in group.windows(2) {
+                uf.union(pair[0], pair[1]);
+            }
+        }
+
+        // Compact roots into dense component indices, deterministically.
+        let mut root_to_component: BTreeMap<usize, usize> = BTreeMap::new();
+        let mut members: Vec<Vec<Serial>> = Vec::new();
+        let mut of_node = BTreeMap::new();
+        for (slot, serial) in serials.iter().enumerate() {
+            let root = uf.find(slot);
+            let component = *root_to_component.entry(root).or_insert_with(|| {
+                members.push(Vec::new());
+                members.len() - 1
+            });
+            members[component].push(*serial);
+            of_node.insert(*serial, component);
+        }
+        Self { of_node, members }
+    }
+
+    pub fn component_of(&self, serial: Serial) -> Option<usize> {
+        self.of_node.get(&serial).copied()
+    }
+
+    /// Member serials of the component containing `serial`, including it.
+    /// Empty if the node is not in this snapshot.
+    pub fn members_with(&self, serial: Serial) -> &[Serial] {
+        match self.component_of(serial) {
+            Some(component) => &self.members[component],
+            None => &[],
+        }
+    }
+
+    pub fn components(&self) -> impl Iterator<Item = &[Serial]> {
+        self.members.iter().map(Vec::as_slice)
+    }
+}
+
+struct UnionFind {
+    parent: Vec<usize>,
+}
+
+impl UnionFind {
+    fn new(len: usize) -> Self {
+        Self {
+            parent: (0..len).collect(),
+        }
+    }
+
+    fn find(&mut self, mut node: usize) -> usize {
+        while self.parent[node] != node {
+            self.parent[node] = self.parent[self.parent[node]];
+            node = self.parent[node];
+        }
+        node
+    }
+
+    fn union(&mut self, a: usize, b: usize) {
+        let (a, b) = (self.find(a), self.find(b));
+        if a != b {
+            // Lowest root wins, so components are deterministic.
+            let (low, high) = if a < b { (a, b) } else { (b, a) };
+            self.parent[high] = low;
+        }
+    }
+}
+
+/// Client objects belonging to an owner component, so sticky taint can be
+/// keyed on every object that constitutes the owner (v3.4 §6.1.3: clear the
+/// entry only once **all** member objects are gone).
+pub fn client_serials_of(
+    snapshot: &GraphSnapshot,
+    keys: &OwnerKeyIndex,
+    nodes: &[Serial],
+) -> Vec<Serial> {
+    let mut out: Vec<Serial> = nodes
+        .iter()
+        // Only nodes for which `client.id` is a *usable* owner key. See
+        // `uses_client_key`: recording a device node's shared session-manager
+        // Client here would defeat exception 2 on the next recompute.
+        .filter(|serial| keys.uses_client_key(**serial))
+        .filter_map(|serial| snapshot.node(*serial))
+        .filter_map(|node| node.props.client_id)
+        // An ambiguous client id means two Clients claim it and we cannot
+        // say which one is ours, so remember both: an entry that recorded
+        // neither could be retired while its owner was still live.
+        .flat_map(|id: GlobalId| snapshot.clients_with_id(id).map(|client| client.serial))
+        .collect();
+    out.sort_unstable();
+    out.dedup();
+    out
+}

src/host/taint/snapshot.rs

@@ -0,0 +1,355 @@
+//! The plain, owned graph model the taint engine reasons over.
+//!
+//! **No PipeWire types appear in this file, by design** (impl plan §4,
+//! phase 2). The registry observer (phase 3) translates live globals into
+//! these structs; every test builds them by hand. Nothing here ever links
+//! against libpipewire.
+//!
+//! Two id-ish things live in this model and confusing them is the bug the
+//! whole file is shaped to prevent:
+//!
+//! - [`Serial`] — `object.serial`, 64-bit, monotonic, **never reused**.
+//!   This is *identity*. Sticky taint is keyed on it.
+//! - [`GlobalId`] — the PipeWire global id, 32-bit and **recycled**. It is
+//!   a *lookup key within one snapshot* and nothing else: links name their
+//!   endpoints with it, nodes name their client with it. It must never
+//!   outlive the snapshot it was read from (design v3.4 §6.1.3).
+
+use std::collections::BTreeMap;
+
+/// `object.serial` — 64-bit, monotonic, never recycled. Identity.
+#[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Debug, Hash)]
+pub struct Serial(pub u64);
+
+/// A PipeWire global id — 32-bit and **recycled**. Snapshot-local lookup
+/// key only; see the module docs.
+#[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Debug, Hash)]
+pub struct GlobalId(pub u32);
+
+/// What a node does with audio, parsed from `media.class`.
+///
+/// Taint is computed at **node** granularity (v3.4 §6.1 edge type 2: the
+/// monitor connection is already a real Link whose output node is the sink
+/// itself, so a node-level walk crosses `app → sink → monitor-reader` for
+/// free). Ports exist in the model for link creation in phase 6 and for the
+/// `port.exclusive` predicate, not for taint.
+#[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Debug, Hash)]
+pub enum MediaRole {
+    /// `Stream/Output/Audio` — an application playing audio. The only
+    /// fan-out candidate.
+    StreamOutput,
+    /// `Stream/Input/Audio` — an application capturing audio.
+    StreamInput,
+    /// `Audio/Sink` — a real or virtual sink.
+    Sink,
+    /// `Audio/Source` — a real or virtual source.
+    Source,
+    /// `Audio/Duplex`. ⚠️ Node granularity smears taint across both roles
+    /// of these; accepted for v1 as fail-closed over-exclusion
+    /// (v3.4 §6.1, edge type 2 caveat).
+    Duplex,
+    /// Anything else, including video and unparseable/absent `media.class`.
+    Other,
+}
+
+impl MediaRole {
+    pub fn parse(media_class: Option<&str>) -> Self {
+        match media_class {
+            Some("Stream/Output/Audio") => Self::StreamOutput,
+            Some("Stream/Input/Audio") => Self::StreamInput,
+            Some("Audio/Sink") => Self::Sink,
+            Some("Audio/Source") => Self::Source,
+            Some("Audio/Duplex") => Self::Duplex,
+            _ => Self::Other,
+        }
+    }
+
+    /// Can this node *receive* audio? This is the gate on the owner bridge:
+    /// taint crosses the intra-process hop only when the owner is actually
+    /// reading tainted audio (v3.4 §6.1.1 — "this client has both an input
+    /// and an output leg ⇒ exclude the output" is the catastrophic rule
+    /// that excludes every app with a microphone).
+    ///
+    /// `Sink` counts: EasyEffects' `ee_sink` is an `Audio/Sink` that
+    /// receives the tainted mix, and its re-emitting leg is joined to it by
+    /// `node.link-group` with no Link between them.
+    pub fn receives_audio(self) -> bool {
+        matches!(self, Self::StreamInput | Self::Sink | Self::Duplex)
+    }
+
+    /// Device-ish nodes — everything that is not a `Stream/*`. Coarse owner
+    /// keys are not allowed to bridge these; see [`super::owner`].
+    pub fn is_device_role(self) -> bool {
+        matches!(self, Self::Sink | Self::Source | Self::Duplex)
+    }
+
+    /// Only `Stream/Output/Audio` nodes are fan-out candidates (v3.4 §6.2).
+    pub fn is_candidate(self) -> bool {
+        matches!(self, Self::StreamOutput)
+    }
+
+    /// Stable machine-readable code for the audit output. Not the raw
+    /// `media.class`: `Other` has no single one, and the audit's codes are a
+    /// contract with the matrix, not with PipeWire.
+    pub fn code(self) -> &'static str {
+        match self {
+            Self::StreamOutput => "stream-output",
+            Self::StreamInput => "stream-input",
+            Self::Sink => "sink",
+            Self::Source => "source",
+            Self::Duplex => "duplex",
+            Self::Other => "other",
+        }
+    }
+}
+
+/// The subset of node properties the engine actually reasons about.
+///
+/// Deliberately a struct of parsed fields rather than a property bag: the
+/// parsing (and its failure modes) belongs at the observer boundary, and a
+/// bag invites `props.get("...")` typos that silently read `None` — which
+/// on this feature means "not tainted".
+#[derive(Clone, Debug, Default, PartialEq, Eq)]
+pub struct NodeProps {
+    /// `peerspeak.owned` is present and **exactly**
+    /// [`super::PEERSPEAK_OWNED_VALUE`] (v3.4 §5.1, tightened by round 10's
+    /// R10-4 — it is not "present and truthy", and the round-10 review found
+    /// this doc still saying so). A correctness mechanism, explicitly *not* a
+    /// security boundary.
+    ///
+    /// ⚠️ **Ownership carrier 1 of 2, so this being `false` does not mean
+    /// "not peerspeak's".** Carrier 2 is the [`NodeSnapshot::name`] prefix
+    /// [`super::PEERSPEAK_OWNED_NODE_PREFIX`], matched as a union in
+    /// `local_root_reason`. Read that function, not this field, to answer
+    /// "is this node owned?".
+    pub peerspeak_owned: bool,
+    /// `pulse.module.id`, parsed as `u64` — never `u32`, per v3.4 §5.2's
+    /// parse-defensively note and the phase 0a truncation bug.
+    pub pulse_module_id: Option<u64>,
+    /// `node.link-group` — owner key 1, and the `echo-cancel-` hazard
+    /// prefix (v3.4 §5.4 / D3).
+    pub link_group: Option<String>,
+    /// `client.id` — owner key 3. A **connection**, not an owner: GStreamer
+    /// opens one per stream (v3.4 §6.1.2, measured refutation).
+    pub client_id: Option<GlobalId>,
+    /// `application.process.id` **on the node** — owner key 4. For
+    /// module-created streams this is pipewire-pulse's own PID, which is
+    /// why [`super::ExclusionCtx::pipewire_pulse_pid`] exists.
+    pub process_id: Option<u32>,
+    /// The stream negotiated an encoded/passthrough format; a second link
+    /// would refuse or corrupt it (v3.4 §6.2).
+    pub passthrough: bool,
+    /// This node is a **passive device node exported by the session
+    /// manager** — a real sound card's sink or source, not something that
+    /// forwards audio.
+    ///
+    /// ⚠️ **A positive high-confidence classification the observer owes, not
+    /// a raw property** (Codex rounds 2–3). PipeWire defines `device.id`
+    /// only as "the Device this node belongs to" and `device.api` as that
+    /// Device's access API; **neither promises the node passively terminates
+    /// audio**, so a card-associated filter can satisfy both. Setting this
+    /// flag *removes* two protections at once — the node's coarse owner keys
+    /// (`owner` exception 2) and its ability to trip the fail-closed
+    /// backstop — so a false positive is a leak, not over-exclusion.
+    ///
+    /// **Phase-3 contract:**
+    /// - Set `true` only on positively-identified passive hardware
+    ///   terminals: a resolved `device.id` on a real backend
+    ///   (`device.api` present) whose `factory.name` is on an **explicit
+    ///   hardware-PCM allowlist** — `api.alsa.pcm.sink`, `api.alsa.pcm.source`,
+    ///   and the equivalent for other real backends (bluez5, v4l2 for the
+    ///   media case) as phase 3 enumerates them — never a filter, loopback,
+    ///   or `support.null-audio-sink` factory. An allowlist, not a
+    ///   substring or a denylist: an unknown factory is not a device.
+    ///   Measured discriminator on the
+    ///   target box: the five ALSA nodes carry `device.api=alsa` +
+    ///   `factory.name=api.alsa.pcm.*` and share `client.id=42`
+    ///   (`WirePlumber [export]`); the three `support.null-audio-sink` nodes
+    ///   carry neither. (`node.physical` was measured **null** on the ALSA
+    ///   nodes here, so it is *not* a usable discriminator — do not rely on
+    ///   it.)
+    /// - **Fail closed: unknown ⇒ `false`.** A node that cannot be
+    ///   positively classified keeps its owner keys and can trip the
+    ///   backstop; both are the safe direction.
+    /// - A node MUST NOT enter a snapshot with this field provisional. If
+    ///   the Device backing a node has not yet been bound, withhold the node
+    ///   and keep the epoch not-ready — otherwise a provisional `false`
+    ///   during not-ready fuses sink and mic on the shared session client
+    ///   and that fusion can persist as sticky over-exclusion (round-3
+    ///   finding 3).
+    ///
+    /// ⚠️ **A false positive is leak-capable — do not treat it as braced.**
+    /// I claimed a mis-classified filter could not leak because its legs
+    /// share a `node.link-group` (strong-key bridge) or trip the unbounded
+    /// backstop. Codex refuted it (round 4): a filter *without* a shared
+    /// strong key, marked `session_device=true`, cannot activate the
+    /// backstop from its reading leg, so a differently-keyed re-emitting leg
+    /// leaks. Those braces catch *some* shapes, not all. The only real
+    /// defence is a correct classifier — hence "positive high-confidence"
+    /// and "fail closed to false" above, without exception.
+    ///
+    /// What it is for: every real device node shares the session manager's
+    /// `client.id`, so coarse owner keys must not bridge them — else
+    /// peerspeak's playback (which taints the default sink every recompute)
+    /// would reach the microphone. See [`super::owner`] exception 2.
+    pub session_device: bool,
+}
+
+#[derive(Clone, Debug, PartialEq, Eq)]
+pub struct NodeSnapshot {
+    pub serial: Serial,
+    pub id: GlobalId,
+    /// `node.name`, for diagnostics and for `pixelpass_capture_*` ancestry
+    /// detection (v3.4 §6.2, cycle prevention).
+    pub name: Option<String>,
+    pub role: MediaRole,
+    pub props: NodeProps,
+}
+
+#[derive(Clone, Copy, PartialEq, Eq, Debug)]
+pub enum PortDirection {
+    In,
+    Out,
+}
+
+#[derive(Clone, Debug, PartialEq, Eq)]
+pub struct PortSnapshot {
+    pub serial: Serial,
+    pub id: GlobalId,
+    /// Owning node, by snapshot-local id.
+    pub node: GlobalId,
+    pub direction: PortDirection,
+    /// `port.exclusive` — fan-out will be refused (v3.4 §6.2).
+    pub exclusive: bool,
+    /// `port.monitor`. Recorded for phase 6 link creation; taint does not
+    /// need it at node granularity.
+    pub monitor: bool,
+}
+
+#[derive(Clone, Debug, PartialEq, Eq)]
+pub struct LinkSnapshot {
+    pub serial: Serial,
+    pub id: GlobalId,
+    /// `link.output.node` — the node audio flows **from**.
+    pub output_node: GlobalId,
+    /// `link.input.node` — the node audio flows **to**.
+    pub input_node: GlobalId,
+    pub output_port: Option<GlobalId>,
+    pub input_port: Option<GlobalId>,
+}
+
+#[derive(Clone, Debug, PartialEq, Eq)]
+pub struct ClientSnapshot {
+    pub serial: Serial,
+    pub id: GlobalId,
+    /// `pipewire.sec.pid` — for Pulse-emulated clients this is
+    /// **pipewire-pulse's** PID, identical across every unrelated app
+    /// (v3.4 §5.2 correction 5). Phase 3 derives the daemon PID from the
+    /// consistency of this value; the engine only consumes the result.
+    pub sec_pid: Option<u32>,
+}
+
+/// How a snapshot-local id resolves.
+#[derive(Clone, Copy, PartialEq, Eq, Debug)]
+pub enum IdLookup {
+    Unique(Serial),
+    /// Two live objects in one snapshot claim the same global id — the
+    /// observer missed a removal, so the recycled id is ambiguous. Every
+    /// edge touching it is treated as unresolved, i.e. fail closed.
+    Ambiguous,
+}
+
+/// One coherent observation of the graph.
+///
+/// Built through [`GraphSnapshot::new`] so the id indexes and the ambiguity
+/// detection cannot be skipped.
+#[derive(Clone, Debug, Default, PartialEq, Eq)]
+pub struct GraphSnapshot {
+    nodes: BTreeMap<Serial, NodeSnapshot>,
+    ports: BTreeMap<Serial, PortSnapshot>,
+    links: BTreeMap<Serial, LinkSnapshot>,
+    clients: BTreeMap<Serial, ClientSnapshot>,
+    node_ids: BTreeMap<GlobalId, IdLookup>,
+    client_ids: BTreeMap<GlobalId, IdLookup>,
+}
+
+impl GraphSnapshot {
+    pub fn new(
+        nodes: Vec<NodeSnapshot>,
+        ports: Vec<PortSnapshot>,
+        links: Vec<LinkSnapshot>,
+        clients: Vec<ClientSnapshot>,
+    ) -> Self {
+        let node_ids = index_ids(nodes.iter().map(|n| (n.id, n.serial)));
+        let client_ids = index_ids(clients.iter().map(|c| (c.id, c.serial)));
+        Self {
+            nodes: nodes.into_iter().map(|n| (n.serial, n)).collect(),
+            ports: ports.into_iter().map(|p| (p.serial, p)).collect(),
+            links: links.into_iter().map(|l| (l.serial, l)).collect(),
+            clients: clients.into_iter().map(|c| (c.serial, c)).collect(),
+            node_ids,
+            client_ids,
+        }
+    }
+
+    pub fn nodes(&self) -> impl Iterator<Item = &NodeSnapshot> {
+        self.nodes.values()
+    }
+
+    pub fn node(&self, serial: Serial) -> Option<&NodeSnapshot> {
+        self.nodes.get(&serial)
+    }
+
+    pub fn links(&self) -> impl Iterator<Item = &LinkSnapshot> {
+        self.links.values()
+    }
+
+    pub fn ports(&self) -> impl Iterator<Item = &PortSnapshot> {
+        self.ports.values()
+    }
+
+    pub fn clients(&self) -> impl Iterator<Item = &ClientSnapshot> {
+        self.clients.values()
+    }
+
+    /// Resolve a snapshot-local node id. `None` means "no such node in this
+    /// snapshot", which for a link endpoint means unresolved ancestry.
+    pub fn node_by_id(&self, id: GlobalId) -> Option<IdLookup> {
+        self.node_ids.get(&id).copied()
+    }
+
+    pub fn client_by_id(&self, id: GlobalId) -> Option<IdLookup> {
+        self.client_ids.get(&id).copied()
+    }
+
+    /// Every node claiming a global id. More than one means the id is
+    /// [`IdLookup::Ambiguous`] and each claimant must be treated as a
+    /// possible endpoint of any link naming it.
+    pub fn nodes_with_id(&self, id: GlobalId) -> impl Iterator<Item = &NodeSnapshot> {
+        self.nodes.values().filter(move |node| node.id == id)
+    }
+
+    /// Every client claiming a global id — same fail-closed reasoning.
+    pub fn clients_with_id(&self, id: GlobalId) -> impl Iterator<Item = &ClientSnapshot> {
+        self.clients.values().filter(move |client| client.id == id)
+    }
+
+    /// Ports belonging to a node, by the node's snapshot-local id.
+    pub fn ports_of(&self, node: GlobalId) -> impl Iterator<Item = &PortSnapshot> {
+        self.ports.values().filter(move |p| p.node == node)
+    }
+}
+
+fn index_ids(entries: impl Iterator<Item = (GlobalId, Serial)>) -> BTreeMap<GlobalId, IdLookup> {
+    let mut out: BTreeMap<GlobalId, IdLookup> = BTreeMap::new();
+    for (id, serial) in entries {
+        out.entry(id)
+            .and_modify(|slot| {
+                if *slot != IdLookup::Unique(serial) {
+                    *slot = IdLookup::Ambiguous;
+                }
+            })
+            .or_insert(IdLookup::Unique(serial));
+    }
+    out
+}

src/interactive.rs

@@ -281,8 +281,10 @@ impl Player {
                 &[
                     // No `--untimed`: it ignores audio timestamps and drifts a
                     // shared video out of sync. Pacing to audio keeps A/V synced.
+                    // Also leave hwdec at the `low-latency` default (software
+                    // decode): forcing `--hwdec=auto` froze some viewers on
+                    // frame 1 while audio kept playing.
                     "--profile=low-latency",
-                    "--hwdec=auto",
                     "--audio-buffer=0.2",
                     "--demuxer-max-bytes=2M",
                     "--demuxer-readahead-secs=0.5",

src/main.rs

@@ -1,5 +1,6 @@
 mod cli;
 mod common;
+mod doctor;
 #[cfg(feature = "gui")]
 mod gui;
 mod host;
@@ -36,6 +37,13 @@ async fn main() -> Result<()> {
         }
     }
 
+    // Diagnostics run before pipewire::init() (they don't need it) and work
+    // regardless of the `gui` feature, so a headless tester can probe their box.
+    if cli.doctor {
+        let relay = common::endpoint::relay_override(cli.relay.as_deref());
+        return doctor::run(relay).await;
+    }
+
     // libpipewire requires global init before any pw_* call. Idempotent;
     // safe to call even when the per-app audio thread never spawns.
     pipewire::init();
@@ -44,6 +52,13 @@ async fn main() -> Result<()> {
         return repair::run().await;
     }
 
+    // Read-only diagnostic: observe the graph, report what the audio-exclusion
+    // engine concludes, create nothing. Placed before the host/viewer dispatch
+    // because it is neither — it shares no screen and connects to no peer.
+    if cli.audit_audio {
+        return host::audit::run::run_standalone().await;
+    }
+
     if cli.reconfigure {
         return interactive::run_reconfigure().await;
     }

src/viewer/mod.rs

@@ -102,7 +102,7 @@ fn print_viewer_banner(url: &str) {
     eprintln!("│  Connected to host. Open the stream in your player:");
     eprintln!("│");
     eprintln!(
-        "│    mpv --profile=low-latency --hwdec=auto --audio-buffer=0.2 --demuxer-max-bytes=2M --demuxer-readahead-secs=0.5 {url}"
+        "│    mpv --profile=low-latency --audio-buffer=0.2 --demuxer-max-bytes=2M --demuxer-readahead-secs=0.5 {url}"
     );
     eprintln!("│    vlc --network-caching=200 --live-caching=200 {url}");
     eprintln!("│");

tests/fixtures/ownership-tag-contract.txt

@@ -0,0 +1,42 @@
+# Screenshare audio exclusion — ownership tagging wire contract.
+#
+# peerspeak PRODUCES these carriers on every audio node it owns; pixelpass
+# CONSUMES them as the primary taint root of the exclusion engine. Neither
+# repo depends on the other, so this file is the contract: it is committed
+# byte-identical in both, and each repo has a test that asserts its own named
+# constants (and, on the producer side, the environment a real child Command
+# would carry) match these values exactly.
+#
+#   peerspeak/tests/fixtures/ownership-tag-contract.txt
+#   pixelpass/tests/fixtures/ownership-tag-contract.txt
+#
+# Pinned by peerspeak docs/screenshare-audio-exclusion-impl-plan.md §3 and
+# docs/screenshare-audio-exclusion-plan.md §5.1 (v3.5). Changing a value here
+# is a cross-repo breaking change: both repos must land in the same session,
+# and the phase 5 matrix must be re-run.
+#
+# Two carriers, matched as a UNION — a node is peerspeak-owned if EITHER
+# matches. Round 8 added the second because a property is invisible to the
+# PipeWire registry `global` event and readable only via a node bind, so the
+# primary taint root must not rest on one observation mechanism alone.
+
+# Carrier 1 — a node property, matched EXACTLY: `prop_value` below is the
+# ONLY spelling the consumer reads as owned. A producer emitting "true", "yes"
+# or "" is NOT owned on this carrier, and only carrier 2 would still catch it.
+#
+# ⚠️ This wording is load-bearing and it CHANGED in round 10. The consumer
+# used to accept any value other than "false"/"0", on the theory that leniency
+# over-excludes and is therefore safe. It is not: leniency buys false-positive
+# exclusion, and it let any process suppress a rival application's audio from
+# the share with a property it did not even have to spell right. Fail-closed
+# on this feature is about ANCESTRY — an unresolvable graph is not eligible —
+# not about parsing.
+prop_key=peerspeak.owned
+prop_value=1
+
+# Carrier 2 — a `node.name` prefix, announced by the registry without a bind.
+# `node.description` is deliberately NOT touched, so mixers still show "mpv".
+# Only the prefix is matched; the rest of the name is for diagnostics.
+node_name_prefix=peerspeak_owned_
+node_name_format=peerspeak_owned_<role>_<pid>
+node_name_example=peerspeak_owned_mpv_31284