feat(audio): add --strict-audio + app_audio route-status events

With --app, pixelpass mirrors the default-sink monitor (whole desktop) until the
chosen app's streams route, and restores that loopback if the app's audio later
stops. That fallback captures everything playing — including a voice call the
sharer is in — so a caller watching the share can hear themselves echoed back
(peerspeak bug A23: the per-app pick alone is best-effort, not a guarantee).

- New --strict-audio flag (HostOpts.strict_audio): with --app, never load the
  default-sink loopback (not at startup, not on LastRoutedStreamGone). The viewer
  hears only the chosen app, and silence when it's quiet — never the rest of the
  desktop. No effect without --app; standalone best-effort behavior is unchanged.
- New app_audio JSON event ({"event":"app_audio","state":"routed"|"lost"}),
  emitted whenever --app is set, so a front-end (peerspeak) can tell when the
  chosen app's audio is actually live vs. dropped and warn accordingly.
- Banner capture summary shows "(strict)" when active.

Unknown-event-tolerant: pixelpass's own --gui child parser skips lines it can't
deserialize, so app_audio doesn't disturb it. 10 tests (+2: wire-shape + banner),
clippy --all-targets clean.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/host/mod.rs

@@ -488,9 +488,51 @@ fn copy_to_clipboard(text: &str) -> bool {
 fn capture_summary(opts: &HostOpts) -> String {
     let mut bits = vec![if opts.window { "window" } else { "fullscreen" }.to_string()];
     if let Some(app) = &opts.app {
-        bits.push(format!("app-audio={app}"));
+        if opts.strict_audio {
+            bits.push(format!("app-audio={app} (strict)"));
+        } else {
+            bits.push(format!("app-audio={app}"));
+        }
     } else {
         bits.push("system-audio".to_string());
     }
     bits.join(" + ")
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::cli::Quality;
+
+    fn opts(app: Option<&str>, strict_audio: bool) -> HostOpts {
+        HostOpts {
+            window: false,
+            app: app.map(str::to_string),
+            strict_audio,
+            display_server: None,
+            quality: Quality::Auto,
+            bitrate: None,
+            framerate: None,
+            max_height: None,
+            no_hwencode: false,
+            max_viewers: None,
+            interactive: false,
+            relay: None,
+        }
+    }
+
+    #[test]
+    fn capture_summary_reflects_audio_mode() {
+        assert_eq!(capture_summary(&opts(None, false)), "fullscreen + system-audio");
+        assert_eq!(
+            capture_summary(&opts(Some("Firefox"), false)),
+            "fullscreen + app-audio=Firefox"
+        );
+        // strict only shows when an app is selected.
+        assert_eq!(
+            capture_summary(&opts(Some("Firefox"), true)),
+            "fullscreen + app-audio=Firefox (strict)"
+        );
+        assert_eq!(capture_summary(&opts(None, true)), "fullscreen + system-audio");
+    }
+}