fix: correct JACK shutdown callback signature and suppress unused parameter warning

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

engine.c

@@ -84,10 +84,10 @@ static int process_callback(jack_nframes_t nframes, void *arg) {
 }
 
 // JACK shutdown callback
-static void shutdown_callback(jack_status_t code, const char *reason, void *arg) {
+static void shutdown_callback(void *arg) {
     Engine *engine = (Engine *)arg;
     engine->running = false;
-    fprintf(stderr, "JACK shutdown: %s\n", reason);
+    fprintf(stderr, "JACK shutdown\n");
 }
 
 int engine_init(Engine *engine, const char *client_name) {

main.c

@@ -8,6 +8,7 @@ static Engine engine;
 static volatile int keep_running = 1;
 
 void signal_handler(int sig) {
+    (void)sig;
     keep_running = 0;
 }