feat: audio - add concurrent mixing, output switching, and device-aware UI

template/docs/syscalls.md

@@ -417,6 +417,7 @@ int audio_pause(int handle);
 int audio_resume(int handle);
 int audio_get_pos(int handle);                  // Playback position
 int audio_get_output(int handle);               // Current output device
+int audio_set_output(int output);                // 0=HDA, 1=Bluetooth
 int audio_bt_status(int handle);                // Bluetooth audio status
 ```