feat: add MIDI stop command and FIFO pipe integration test

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

src/midi.c

@@ -63,6 +63,11 @@ void midi_handle_events(void *port_buffer, jack_nframes_t nframes) {
               command_t cmd = { .type = CMD_UNBIND, .channel = -1, .data = 0 };
               queue_push(&cmd_queue, cmd);
             } break;
+            case 65:
+            {
+              command_t cmd = { .type = CMD_STOP, .channel = -1, .data = 0 };
+              queue_push(&cmd_queue, cmd);
+            } break;
             default:
               break;
             }