Fix Ctrl+C not terminating the C backend

Two issues:
- signal() on Linux sets SA_RESTART by default, causing sleep(1) to be
  restarted after the signal handler runs. Switch to sigaction() with no
  SA_RESTART flag and replace sleep(1) with pause() so the main thread
  wakes and exits immediately on SIGINT/SIGTERM.
- close(sock_fd) is not guaranteed to unblock another thread blocked in
  accept() on that fd. Add shutdown(SHUT_RDWR) before close() in
  socket_server_stop() for both the listening socket and the client
  socket, which reliably wakes blocked accept() and read() calls.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

c-backend/src/main.c

@@ -164,7 +164,7 @@ static void on_disconnect(void *ctx) {
 
 /* ── Signal handler ───────────────────────────────────────────────── */
 
-static volatile int g_running = 1;
+static volatile sig_atomic_t g_running = 1;
 static void on_signal(int sig) { (void)sig; g_running = 0; }
 
 /* ── Entry point ──────────────────────────────────────────────────── */
@@ -190,12 +190,14 @@ int main(int argc, char *argv[]) {
 		return 1;
 	}
 
-	signal(SIGINT,  on_signal);
+	struct sigaction sa = { .sa_handler = on_signal, .sa_flags = 0 };
-	signal(SIGTERM, on_signal);
+	sigemptyset(&sa.sa_mask);
+	sigaction(SIGINT,  &sa, NULL);
+	sigaction(SIGTERM, &sa, NULL);
 
 	fprintf(stderr, "midi-sequencer ready, waiting for node client\n");
 	while (g_running) {
-		sleep(1);
+		pause();  /* sleep until any signal; returns immediately on SIGINT/SIGTERM */
 	}
 
 	fprintf(stderr, "midi-sequencer shutting down\n");

c-backend/src/socket_server.c

@@ -127,9 +127,15 @@ int socket_server_start(Socket_Server *srv, const char *path,
 
 void socket_server_stop(Socket_Server *srv) {
 	atomic_store(&srv->running, 0);
+	/* shutdown before close to reliably unblock accept() in accept_thread */
+	shutdown(srv->sock_fd, SHUT_RDWR);
 	close(srv->sock_fd);
 	int fd = atomic_exchange(&srv->client_fd, -1);
-	if (fd >= 0) close(fd);
+	if (fd >= 0) {
+		/* shutdown before close to reliably unblock read() in read_thread */
+		shutdown(fd, SHUT_RDWR);
+		close(fd);
+	}
 	pthread_join(srv->accept_thread, NULL);
 	pthread_join(srv->read_thread, NULL);
 	pthread_mutex_destroy(&srv->write_mutex);