perf: do not hold the audio buffer lock during disk writes

_flush_buffer_to_file wrote (and for FLAC, encoded) every chunk while
holding buffer_lock, blocking the capture callback for the duration of
each disk flush. Swap the buffer out under the lock and write outside.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

isr.py

@@ -714,11 +714,12 @@ class SoundcardRecorder(BaseRecorder):
         if self.current_file is None:
             return
 
+        # Swap the buffer under the lock, write outside it — disk writes (and
+        # FLAC encoding) must not block the capture callback.
         with self.buffer_lock:
-            if self.audio_buffer:
-                for data in self.audio_buffer:
+            buffered, self.audio_buffer = self.audio_buffer, []
+        for data in buffered:
             self.current_file.write(data)
-                self.audio_buffer.clear()
 
     def close_current_file(self):
         """Flush any buffered audio, then close the current recording file."""