fix(host): self-heal capture loss + audio-thread death mid-session

Two steady-state faults previously bubbled a bare `?` to conn.close / silently
muted the rest of a session. Recover in place instead.

#4 — capture loss (virtual_stream): a mid-session capture stall/disconnect
(`try_latest` Err: PipeWire/compositor thread ended, virtual output gone) ended
the whole session — and the native client has no reconnect path, so it had to
cold-restart the handshake. Now rebuild the pipeline IN PLACE at the current
mode via build_pipeline_with_retry (same primitive the mode/session switch uses),
force a keyframe, and only propagate when the bounded retry is exhausted. A
consecutive-rebuild cap stops a flapping source from looping the client through
endless cold IDRs. Track the live mode so a rebuild after a mode switch targets
the right mode (also fixes the session-switch rebuild using the stale mode).

#3 — native audio thread (audio_thread): broke the loop on ANY next_chunk Err,
spawned once per session and never restarted, so a transient 5 s quiet-sink
timeout permanently muted a multi-hour session. Make a quiet sink return an empty
chunk (not an Err) in both backends so only a genuinely dead capture thread is an
Err, and reopen-with-backoff (INJECTOR_REOPEN_BACKOFF) on death, keeping the Opus
encoder + monotonic seq. Documents the next_chunk contract; also makes the
GameStream audio sender survive quiet sinks for free.

Resolves reliability backlog #3 and #4.

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

crates/punktfunk-host/src/audio/linux.rs

@@ -65,7 +65,9 @@ impl AudioCapturer for PwAudioCapturer {
     fn next_chunk(&mut self) -> Result<Vec<f32>> {
         match self.chunks.recv_timeout(Duration::from_secs(5)) {
             Ok(c) => Ok(c),
-            Err(RecvTimeoutError::Timeout) => Err(anyhow!("no PipeWire audio within 5s")),
+            // A quiet sink (paused game, idle desktop) is NOT a failure — return an empty chunk so the
+            // caller keeps the capturer alive. Only a dead capture thread is an Err (→ caller reopens).
+            Err(RecvTimeoutError::Timeout) => Ok(Vec::new()),
             Err(RecvTimeoutError::Disconnected) => Err(anyhow!("pipewire audio thread ended")),
         }
     }

crates/punktfunk-host/src/audio/wasapi_cap.rs

@@ -77,7 +77,9 @@ impl AudioCapturer for WasapiLoopbackCapturer {
     fn next_chunk(&mut self) -> Result<Vec<f32>> {
         match self.chunks.recv_timeout(Duration::from_secs(5)) {
             Ok(c) => Ok(c),
-            Err(RecvTimeoutError::Timeout) => Err(anyhow!("no WASAPI audio within 5s")),
+            // A quiet sink is NOT a failure — return an empty chunk so the caller keeps the capturer
+            // alive. Only a dead capture thread is an Err (→ caller reopens). Matches the Linux path.
+            Err(RecvTimeoutError::Timeout) => Ok(Vec::new()),
             Err(RecvTimeoutError::Disconnected) => Err(anyhow!("wasapi audio thread ended")),
         }
     }