Phase 1 foundation: CLI, iroh tunnel, lazy capture wiring

Scaffolding for PixelPass per ~/Documents/p2p-screenshare-plan.md §7
"Phase 1 — MVP". The full QUIC tunnel handshake works end-to-end
(verified locally: host generates ticket, viewer dials through iroh's
relay, open_bi succeeds, lazy capture is wired correctly).

What's implemented:

- Cargo project with deps locked: iroh 1.0.0-rc.0, iroh-tickets,
  tokio, clap, ashpd, pipewire-rs, x11rb, ashpd, anyhow, thiserror,
  tracing, nix, directories, uuid.
- src/cli.rs: complete clap surface per plan §6 (--window, --app,
  --mic, --display-server, --bitrate, --framerate, --no-hwencode,
  --low-latency, --port, --verbose, --repair).
- Mode dispatch in main.rs: EndpointTicket::from_str is the
  authoritative check; no regex / heuristics.
- common/display.rs: WAYLAND_DISPLAY → DISPLAY → XDG_SESSION_TYPE
  precedence with --display-server override.
- common/deps.rs: per-distro install hints (pacman/apt/dnf/zypper)
  parsing /etc/os-release.
- common/alpn.rs: ALPN = b"pixelpass/0".
- common/tunnel.rs: generic bidirectional bridge between an iroh
  bi-stream and any AsyncRead+AsyncWrite (typically a TCP socket).
- common/signal.rs: ctrl-c -> CancellationToken; second ctrl-c hard
  exit.
- host/mod.rs: build Endpoint, generate ticket, print banner, await
  first peer (lazy — no ffmpeg until peer connects), accept_bi,
  spawn capture, bridge to localhost ffmpeg HTTP listener.
- host/capture.rs: stub returning Phase-2 error; the place X11
  x11grab and Wayland ashpd+gst pipelines will land.
- viewer/mod.rs: Endpoint, connect with ALPN, open_bi, TcpListener
  on 127.0.0.1, print copy-ready mpv/vlc commands, bridge.
- repair.rs: stub for --repair PipeWire scan.

iroh 1.0-rc renamed Node* -> Endpoint* and moved EndpointTicket into
a sibling crate (iroh-tickets); no design impact. Plan still locked.

src/common/deps.rs

@@ -0,0 +1,67 @@
+use anyhow::{Result, bail};
+use std::path::PathBuf;
+
+use crate::common::display::DisplayServer;
+
+pub fn check_host_binaries(display: DisplayServer) -> Result<()> {
+    require("ffmpeg")?;
+    if display == DisplayServer::Wayland {
+        require("gst-launch-1.0")?;
+    }
+    Ok(())
+}
+
+fn require(bin: &str) -> Result<PathBuf> {
+    match which(bin) {
+        Some(p) => Ok(p),
+        None => {
+            let hint = install_hint(bin);
+            bail!("`{bin}` not found on PATH.\n{hint}");
+        }
+    }
+}
+
+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);
+        if candidate.is_file() {
+            return Some(candidate);
+        }
+    }
+    None
+}
+
+fn install_hint(bin: &str) -> String {
+    let distro = detect_distro();
+    let pkg = match bin {
+        "ffmpeg" => "ffmpeg",
+        "gst-launch-1.0" => match distro.as_deref() {
+            Some("arch" | "cachyos" | "manjaro" | "endeavouros") => "gst-plugins-base gst-plugins-good",
+            Some("debian" | "ubuntu" | "pop" | "linuxmint") => "gstreamer1.0-tools gstreamer1.0-plugins-good",
+            Some("fedora" | "nobara") => "gstreamer1-plugins-good",
+            _ => "gstreamer (and tools / good-plugins)",
+        },
+        _ => bin,
+    };
+
+    let cmd = match distro.as_deref() {
+        Some("arch" | "cachyos" | "manjaro" | "endeavouros") => format!("sudo pacman -S {pkg}"),
+        Some("debian" | "ubuntu" | "pop" | "linuxmint") => format!("sudo apt install {pkg}"),
+        Some("fedora" | "nobara") => format!("sudo dnf install {pkg}"),
+        Some("opensuse" | "opensuse-tumbleweed" | "opensuse-leap") => format!("sudo zypper install {pkg}"),
+        _ => format!("install the `{pkg}` package via your distro's package manager"),
+    };
+
+    format!("Install hint: {cmd}")
+}
+
+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=") {
+            return Some(rest.trim_matches('"').to_lowercase());
+        }
+    }
+    None
+}