feat: add unbind command (note 63) to reset bind_channel to 0

Co-authored-by: aider (deepseek/deepseek-reasoner) <aider@aider.chat>

src/midi.c

@@ -59,6 +59,9 @@ void midi_handle_events(void *port_buffer, jack_nframes_t nframes)
                                 }
                             }
                             break;
+                        case 63: /* unbind – reset bind to channel 0 */
+                            atomic_store(&bind_channel, 0);
+                            break;
                         default:
                             break;
                         }