feat: audio stack working — pulseaudio + x6200_control + voice_rec

- Add pulseaudio in system mode
- Add alsactl restore with correct mixer state from Aether
- Add x6200-control package (I2C hardware control library)
- Audio chain: pulseaudio -> x6200_control_init -> voice_rec -> speaker

br2_external/Config.in

@@ -1,7 +1,5 @@
 menu "Mestre"
 
-# Custom packages will be sourced here as they are added.
-# Each line should look like:
-#     source "$BR2_EXTERNAL_MESTRE_PATH/package/<name>/Config.in"
+source "$BR2_EXTERNAL_MESTRE_PATH/package/x6200-control/Config.in"
 
 endmenu