fix: add missing stdarg.h include and jack_midi_send availability check

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

tests/integration.c

@@ -4,6 +4,7 @@
 #include <signal.h>
 #include <sys/wait.h>
 #include <string.h>
+#include <stdarg.h>
 
 /*
  * Integration test for the JACK looper.
@@ -52,6 +53,14 @@ int main(void) {
     printf("Waiting for looper (pid %d) to register ports...\n", (int)pid);
     sleep(3);  /* generous, JACK ports must be visible */
 
+    /* check jack_midi_send availability */
+    if (system("which jack_midi_send >/dev/null 2>&1") != 0) {
+        fprintf(stderr, "FATAL: jack_midi_send not available (install jack‑tools)\n");
+        kill(pid, SIGTERM);
+        waitpid(pid, NULL, 0);
+        return 1;
+    }
+
     /* 3. send note‑on messages (toggle state machine) */
     printf("Sending first note‑on  → IDLE → RECORD\n");
     run_cmd("jack_midi_send -c looper:control -m '90 01 7f' 2>/dev/null || echo 'jack_midi_send not available'");