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/common/output.rs

@@ -72,7 +72,7 @@ pub enum CaptureState {
     Stopped,
 }
 
-#[derive(Serialize, Clone, Copy)]
+#[derive(Serialize, Clone, Copy, Debug, PartialEq, Eq)]
 #[serde(rename_all = "snake_case")]
 pub enum AppAudioState {
     Routed,