feat: M2 teardown — persistent capturers for clean reconnects

Disconnect/reconnect now works reliably. Previously each stream spawned its own
portal+PipeWire (and PipeWire audio) capture threads and never stopped them, so a
reconnect opened a SECOND screencast session that conflicted with the leaked
first one ("no PipeWire frame within 10s" → black screen on reconnect).

- The screen capturer and audio capturer are now persistent, held in AppState and
  reused across streams (created on the first stream). One screencast session for
  the host's lifetime → no conflict, and instant reconnect (no re-handshake).
  Verified live: 3 stream cycles, 1 create + 2 "reusing capturer", clean every time.
- Capturer::set_active gates the (5K, ~1.3 GB/s) de-pad copy to active streams, so
  the persistent video capturer is nearly free while idle between streams.
- AudioCapturer::drain discards buffered chunks on reuse so the client never hears
  stale audio captured while idle.
- stream.rs / gamestream/audio.rs split into a borrow-the-capturer wrapper + the
  encode/send body, so the capturer is always returned to its slot on exit.

This holds whether the client reconnects via /resume (Moonlight's "running →
play/continue") or a fresh /launch — both re-run RTSP PLAY → a new stream cycle.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/lumen-host/src/audio.rs

@@ -15,6 +15,10 @@ pub trait AudioCapturer: Send {
     /// Block until the next chunk of interleaved samples is available (variable size). The
     /// caller reframes into fixed Opus frames.
     fn next_chunk(&mut self) -> Result<Vec<f32>>;
+
+    /// Discard any buffered chunks (called when a persistent capturer is reused for a new
+    /// stream, so the client doesn't hear stale audio captured while idle). Default: no-op.
+    fn drain(&mut self) {}
 }
 
 /// Open a live capturer for the default sink monitor (system output) via PipeWire.