fix: declare missing variables in process_callback

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

engine.c

@@ -40,6 +40,8 @@ static int process_callback(jack_nframes_t nframes, void *arg) {
     transport_process(engine->transport, nframes, midi_clock_buf, midi_out_buf);
     
     // Process control channel MIDI input (clip triggers)
+    int event_index;
+    jack_midi_event_t midi_event;
     event_index = 0;
     while (jack_midi_event_get(&midi_event, midi_in_buf, event_index) == 0) {
         event_index++;