feat: implement MIDI clock transport control in process callback

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

src/main.c

@@ -63,7 +63,32 @@ static int process(jack_nframes_t nframes, void *arg)
         }
     }
 
-    // midi clock port is registered but not processed here
+    // Process MIDI clock messages for transport control (start/stop)
+    void *midi_clock_buf = jack_port_get_buffer(midi_clock_port, nframes);
+    if (midi_clock_buf) {
+        jack_nframes_t n_clock_events = jack_midi_get_event_count(midi_clock_buf);
+        jack_midi_event_t cev;
+        for (jack_nframes_t j = 0; j < n_clock_events; j++) {
+            if (jack_midi_event_get(&cev, midi_clock_buf, j) == 0) {
+                if (cev.size >= 1) {
+                    unsigned char msg = cev.buffer[0];
+                    // real-time messages: Start (0xFA), Stop (0xFC), Continue (0xFB)
+                    if (msg == 0xFA) { // Start transport -> begin recording if idle
+                        if (current_state == STATE_IDLE) {
+                            current_state = STATE_RECORD;
+                        }
+                    } else if (msg == 0xFC) { // Stop transport -> return to IDLE
+                        current_state = STATE_IDLE;
+                    } else if (msg == 0xFB) { // Continue transport -> resume looping (if paused)
+                        if (current_state == STATE_PAUSED) {
+                            current_state = STATE_LOOPING;
+                        }
+                    }
+                }
+            }
+        }
+    }
+
     return 0;
 }