fix: persistent audio stream and silence detection improvements

- Keep InputStream alive across recordings to avoid CoreAudio hang on
  repeated open/close cycles on macOS.  New _ensure_stream() creates the
  stream once; start()/stop()/cancel() only toggle frame collection.
- Add _close_stream_with_timeout() with daemon thread to prevent
  stream.stop()/close() from blocking indefinitely.
- Add generation counter to detect stale stream-open completions after
  cancel or restart.
- Run recorder.cancel() in background thread from Ctrl+C handler to
  keep the event loop responsive.
- Add shutdown() method called on /voice off to release audio resources.
- Fix silence timer reset during active speech: use dip tolerance for
  _resume_start tracker so natural speech pauses (< 0.3s) don't prevent
  the silence timer from being reset.
- Update tests to match persistent stream behavior.

tests/tools/test_voice_mode.py

@@ -235,8 +235,9 @@ class TestAudioRecorderCancel:
 
         assert recorder.is_recording is False
         assert recorder._frames == []
-        mock_stream.stop.assert_called_once()
-        mock_stream.close.assert_called_once()
+        # Stream is kept alive (persistent) — cancel() does NOT close it.
+        mock_stream.stop.assert_not_called()
+        mock_stream.close.assert_not_called()
 
     def test_cancel_when_not_recording_is_safe(self):
         from tools.voice_mode import AudioRecorder