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.rs

@@ -16,7 +16,10 @@ pub const CHANNELS: usize = 2;
 /// falls behind).
 pub trait AudioCapturer: Send {
     /// Block until the next chunk of interleaved samples is available (variable size). The
-    /// caller reframes into fixed Opus frames.
+    /// caller reframes into fixed Opus frames. An **empty** chunk means "no samples right now"
+    /// (e.g. a quiet sink that hit the internal idle timeout) — NOT an error: the caller keeps the
+    /// capturer. `Err` is reserved for a genuinely dead capture thread, signalling the caller to
+    /// reopen.
     fn next_chunk(&mut self) -> Result<Vec<f32>>;
 
     /// The interleaved channel count this capturer delivers (what it was opened with).