host/audit: name the owner key the bridge resolved on

Impl plan section 5.1 row 1 asserts "reason = owner bridge, naming the key",
and the record could not express that: Reason::code collapses
TaintedOwnerBridge { key } to one string, so an exclusion that arrived across a
named owner key was indistinguishable from one that arrived by an incidental
link walk reaching the same verdict. Telling those apart is the entire point of
the row.

OwnerKey::code already documented itself as ending up in the phase 5 audit
output; it was simply never wired to it. Adds owner_key to AuditRow and
TaintRow, omitted when absent, and absent is meaningful: a bridge whose tainted
member shared no key directly reached the node transitively, so there is no
single key to name and naming one would be a false diagnosis.

Read-only and diagnostic-only. New test mutation-verified (stubbing the key to
None fails it); 221 tests green, clippy clean.

src/host/audit/mod.rs

@@ -74,8 +74,9 @@ use serde::Serialize;
 
 use crate::host::aec::{AecConfig, AecState, AecValidator};
 use crate::host::observer::{EventKind, Millis, Projection, Readiness};
+use crate::host::taint::owner::OwnerKey;
 use crate::host::taint::snapshot::Serial;
-use crate::host::taint::{Decisions, Eligibility, ExclusionCtx, StickyState, evaluate};
+use crate::host::taint::{Decisions, Eligibility, ExclusionCtx, Reason, 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
@@ -181,6 +182,16 @@ pub struct AuditRow {
     pub eligible: bool,
     #[serde(skip_serializing_if = "Option::is_none")]
     pub reason: Option<&'static str>,
+    /// The owner key that carried the taint across, when the reason is
+    /// `tainted-owner-bridge` *and* the tainted member shared a key directly.
+    ///
+    /// §5.1 row 1 asserts "reason = owner bridge, **naming the key**" — the
+    /// point being that the exclusion is provably the owner bridge on a
+    /// specific key rather than an incidental link walk that happens to reach
+    /// the same verdict. [`Reason::code`] collapses the payload, so without
+    /// this field that row cannot be asserted from the record at all.
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub owner_key: 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,
@@ -195,9 +206,25 @@ pub struct TaintRow {
     pub serial: u64,
     pub name: Option<String>,
     pub reason: &'static str,
+    /// As [`AuditRow::owner_key`]. Present here too because the bridge that
+    /// matters for a row's diagnosis is often on a non-candidate node.
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub owner_key: Option<&'static str>,
     pub sticky: bool,
 }
 
+/// The owner key a `tainted-owner-bridge` reason resolved on, if it named one.
+///
+/// `None` for every other reason, and also for a bridge whose tainted member
+/// shared no key *directly* — the taint reached it transitively, so there is no
+/// single key to name and inventing one would be a false diagnosis.
+fn owner_key_of(reason: Reason) -> Option<&'static str> {
+    match reason {
+        Reason::TaintedOwnerBridge { key } => key.map(OwnerKey::code),
+        _ => None,
+    }
+}
+
 /// 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
@@ -424,11 +451,13 @@ fn build_body(
             // 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),
+            let (eligible, reason, owner_key, sticky) = match decision.eligibility {
+                Eligibility::NotEligible { reason, sticky } => {
+                    (false, Some(reason.code()), owner_key_of(reason), sticky)
+                }
                 Eligibility::Eligible => match gate_reason {
-                    Some(gate) => (false, Some(gate.code()), false),
-                    None => (true, None, false),
+                    Some(gate) => (false, Some(gate.code()), None, false),
+                    None => (true, None, None, false),
                 },
             };
             AuditRow {
@@ -436,6 +465,7 @@ fn build_body(
                 name: decision.name.clone(),
                 eligible,
                 reason,
+                owner_key,
                 sticky,
             }
         })
@@ -450,6 +480,7 @@ fn build_body(
             serial: serial.0,
             name: node_name(projection, serial),
             reason: entry.reason.code(),
+            owner_key: owner_key_of(entry.reason),
             sticky: entry.sticky,
         })
         .collect();

src/host/audit/tests.rs

@@ -529,6 +529,53 @@ fn row_1_owner_bridge_forwarder_with_an_untainted_control() {
     );
 }
 
+/// §5.1 row 1's other half: the record must **name the key** the bridge
+/// resolved on, not merely say "owner bridge".
+///
+/// Without this the row is unassertable from the record: `Reason::code`
+/// collapses `TaintedOwnerBridge { key }` to one string, so an exclusion that
+/// arrived by an incidental link walk and one that arrived across a named owner
+/// key are indistinguishable — and the row exists precisely to tell them apart.
+/// The fixture's forwarder legs are joined by `pulse.module.id`, so that is the
+/// key that must be reported.
+#[test]
+fn row_1_names_the_owner_key_the_bridge_resolved_on() {
+    let mut graph = Graph::new();
+    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 body = observe(&mut auditor_off(), &ready(graph.build()), 0)
+        .record
+        .body;
+
+    let playback = body
+        .candidates
+        .iter()
+        .find(|row| row.name.as_deref() == Some("tainted-loopback-playback"))
+        .expect("the forwarder's playback leg is a candidate");
+    assert_eq!(playback.reason, Some("tainted-owner-bridge"));
+    assert_eq!(
+        playback.owner_key,
+        Some("pulse.module.id"),
+        "the bridge key must be named in the record, not collapsed into the reason code"
+    );
+
+    // And it stays absent everywhere it would be a false diagnosis: the tag
+    // exclusion is not a bridge at all.
+    let call_name = owned_name("peerspeak-call", 200);
+    let tagged = body
+        .candidates
+        .iter()
+        .find(|row| row.name.as_deref() == Some(call_name.as_str()))
+        .expect("the tagged call playback is a candidate");
+    assert_eq!(tagged.reason, Some("peerspeak-owned"));
+    assert_eq!(tagged.owner_key, None);
+}
+
 /// §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