fix: add debug print to process_callback to verify it runs

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

engine.c

@@ -13,6 +13,13 @@ static int process_callback(jack_nframes_t nframes, void *arg) {
     Engine *engine = (Engine *)arg;
     if (!engine || !engine->dispatch) return 0;
     
+    // Debug: print every 50th callback
+    static int cb_count = 0;
+    if (cb_count++ % 50 == 0) {
+        fprintf(stdout, "LOOPER_CB: nframes=%u, dispatch=%p, state=%p\n", 
+                nframes, (void*)engine->dispatch, (void*)engine->state);
+    }
+    
     jack_default_audio_sample_t *audio_in[MAX_CHANNELS];
     jack_default_audio_sample_t *audio_out[MAX_CHANNELS];