The handle_voice_record key binding runs in prompt_toolkit's event-loop
thread. When silence auto-stopped recording, _voice_recording was False
but recorder.stop() still held AudioRecorder._lock. A concurrent Ctrl+B
press entered the START path and blocked on that lock, freezing all
keyboard input.
Three changes:
- Set _voice_processing atomically with _voice_recording=False in
_voice_stop_and_transcribe to close the race window
- Add _voice_processing guard in the START path to prevent starting
while stop/transcribe is still running
- Dispatch _voice_start_recording to a daemon thread so play_beep
(sd.wait) and AudioRecorder.start (lock acquire) never block the
event loop
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