Fix HDMI hotplug crash by releasing mutex during blocking ALSA I/O

When HDMI is unplugged during active audio capture, the blocking
snd_pcm_readi() call was holding the mutex, preventing clean shutdown.
This caused snd_pcm_drop() to race with the blocking read, leading to
undefined behavior and crashes.

Solution mirrors PiKVM's approach:
- Release mutex before snd_pcm_readi()/snd_pcm_writei()
- Reacquire mutex after I/O completes
- Verify handle and stop flag before proceeding

This allows snd_pcm_drop() to immediately abort pending I/O when the
device is closed, ensuring clean shutdown during HDMI hotplug events.

internal/audio/c/audio.c

@@ -673,10 +673,16 @@ retry_read:
 
 	snd_pcm_t *handle = pcm_capture_handle;
 
-	// Read from hardware at hardware sample rate
+	// Release mutex before blocking I/O to allow clean shutdown
+	pthread_mutex_unlock(&capture_mutex);
+
+	// Read from hardware at hardware sample rate (blocking call, no mutex held)
 	pcm_rc = snd_pcm_readi(handle, pcm_hw_buffer, hardware_frame_size);
 
-	if (handle != pcm_capture_handle) {
+	// Reacquire mutex and verify device wasn't closed during read
+	pthread_mutex_lock(&capture_mutex);
+
+	if (handle != pcm_capture_handle || atomic_load(&capture_stop_requested)) {
 		pthread_mutex_unlock(&capture_mutex);
 		return -1;
 	}
@@ -884,9 +890,17 @@ retry_write:
 	}
 
 	snd_pcm_t *handle = pcm_playback_handle;
+
+	// Release mutex before blocking I/O to allow clean shutdown
+	pthread_mutex_unlock(&playback_mutex);
+
+	// Write to hardware (blocking call, no mutex held)
 	pcm_rc = snd_pcm_writei(handle, pcm_buffer, pcm_frames);
 
-	if (handle != pcm_playback_handle) {
+	// Reacquire mutex and verify device wasn't closed during write
+	pthread_mutex_lock(&playback_mutex);
+
+	if (handle != pcm_playback_handle || atomic_load(&playback_stop_requested)) {
 		pthread_mutex_unlock(&playback_mutex);
 		return -1;
 	}