audio: replace struct pack/unpack with array module in _apply_gain

Eliminates format string construction and intermediate tuple/list
allocations. Also drains stale frames from queues on stop().

src/tuimble/audio.py

@@ -7,9 +7,9 @@ Playback path: pymumble (decodes) -> raw PCM -> queue -> speakers.
 
 from __future__ import annotations
 
+import array
 import logging
 import queue
-import struct
 
 log = logging.getLogger(__name__)
 
@@ -21,13 +21,13 @@ DTYPE = "int16"
 
 def _apply_gain(pcm: bytes, gain: float) -> bytes:
     """Scale int16 PCM samples by gain factor with clipping."""
-    n = len(pcm) // 2
-    if n == 0:
+    if len(pcm) < 2:
         return pcm
-    fmt = f"<{n}h"
-    samples = struct.unpack(fmt, pcm[: n * 2])
-    scaled = [max(-32768, min(32767, int(s * gain))) for s in samples]
-    return struct.pack(fmt, *scaled)
+    samples = array.array("h")
+    samples.frombytes(pcm[: len(pcm) & ~1])
+    for i in range(len(samples)):
+        samples[i] = max(-32768, min(32767, int(samples[i] * gain)))
+    return samples.tobytes()
 
 
 class AudioPipeline:
@@ -82,13 +82,19 @@ class AudioPipeline:
         log.info("audio pipeline started (rate=%d)", self._sample_rate)
 
     def stop(self):
-        """Close audio streams."""
+        """Close audio streams and drain stale frames."""
         for stream in (self._input_stream, self._output_stream):
             if stream is not None:
                 stream.stop()
                 stream.close()
         self._input_stream = None
         self._output_stream = None
+        for q in (self._capture_queue, self._playback_queue):
+            while not q.empty():
+                try:
+                    q.get_nowait()
+                except queue.Empty:
+                    break
         log.info("audio pipeline stopped")
 
     @property
@@ -154,9 +160,15 @@ class AudioPipeline:
         except queue.Empty:
             outdata[:] = b"\x00" * len(outdata)
 
-    def get_capture_frame(self) -> bytes | None:
-        """Retrieve next captured PCM frame for transmission."""
+    def get_capture_frame(self, timeout: float = 0.0) -> bytes | None:
+        """Retrieve next captured PCM frame for transmission.
+
+        Args:
+            timeout: Seconds to wait for a frame.  0 returns immediately.
+        """
         try:
+            if timeout > 0:
+                return self._capture_queue.get(timeout=timeout)
             return self._capture_queue.get_nowait()
         except queue.Empty:
             return None