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/linux.rs

@@ -39,6 +39,10 @@ impl AudioCapturer for PwAudioCapturer {
             Err(RecvTimeoutError::Disconnected) => Err(anyhow!("pipewire audio thread ended")),
         }
     }
+
+    fn drain(&mut self) {
+        while self.chunks.try_recv().is_ok() {}
+    }
 }
 
 fn pw_thread(tx: std::sync::mpsc::SyncSender<Vec<f32>>) -> Result<()> {