fix: use engine->carla_host instead of state.carla_host in process_callback

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

engine.c

@@ -159,7 +159,7 @@ static int process_callback(jack_nframes_t nframes, void *arg) {
         }
         
         // Process through Carla rack
-        carla_process(&state.carla_host, ch, rack_in, rack_out, nframes);
+        carla_process(&engine->carla_host, ch, rack_in, rack_out, nframes);
         
         // Copy to output
         memcpy(audio_out[ch], rack_out, nframes * sizeof(jack_default_audio_sample_t));