host/audio: emit initial app_audio "lost" at strict capture start (A23 P2/F1)

In strict per-app mode the default-sink loopback is suppressed, so until the
chosen app's first stream routes the viewer hears silence. Previously no event
fired for an app that never routed (`lost` only fires on an N→0 transition
after a prior route), so peerspeak couldn't warn — the share looked normal but
was silent. Emit a `lost` at capture start (lazy, on first viewer) when, and
only when, `--app` + `--strict-audio` are both set; whole-desktop and
best-effort modes keep audio flowing via the loopback and emit nothing.

Factored the emit decision into the pure, unit-tested `initial_app_audio_state`;
derive Debug/PartialEq/Eq on AppAudioState so it can be asserted on.

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

src/host/audio.rs

@@ -165,6 +165,16 @@ impl Routing {
             routing.event_task = Some(event_task);
         }
 
+        // Strict per-app mode suppresses the default-sink loopback, so until the
+        // chosen app's first stream routes the viewer hears *silence*. Emit an
+        // initial `lost` at capture start (capture is lazy — this runs on the
+        // first viewer) so the front-end can warn from the outset rather than
+        // only after an app that *was* routed later stops (audit A23 P2/F1):
+        // `LastRoutedStreamGone`→`lost` never fires for an app that never routed.
+        if let Some(state) = initial_app_audio_state(opts) {
+            crate::common::output::emit(crate::common::output::Event::AppAudio { state });
+        }
+
         Ok(routing)
     }
 
@@ -206,6 +216,16 @@ impl Drop for Routing {
     }
 }
 
+/// The app-audio state to announce at capture start, if any. Only strict per-app
+/// mode warrants one: there the loopback is suppressed, so the viewer hears
+/// silence until the chosen app's first stream routes — surface that as an
+/// initial `lost`. In every other mode (whole-desktop, or best-effort app
+/// filtering) the loopback keeps audio flowing from the outset, so there is no
+/// initial gap to report. Pure: no I/O, so the emit decision is unit-testable.
+pub(super) fn initial_app_audio_state(opts: &HostOpts) -> Option<crate::common::output::AppAudioState> {
+    (opts.app.is_some() && opts.strict_audio).then_some(crate::common::output::AppAudioState::Lost)
+}
+
 // ──────────────────────────────────────────────────────────────────────
 // App enumeration (interactive picker source)
 // ──────────────────────────────────────────────────────────────────────