fix: multiple issues preventing NAM from surviving rate/period changes

Root causes of 'changing bit rate / sample rate kills NAM':

1. Zombie jackd from stale second Python process — when stop_jack()
   kills jackd, the other process's auto-restart respawns jackd with
   OLD config between stop and start. start_jack() sees a running
   jackd and returns True immediately without verifying it's actually
   accepting connections.

2. Zombie Python process from incomplete service restart — two
   main.py instances running, each with its own AudioSystem and
   JackAudioClient. Profile change kills one JACK but the other
   respawns it. jack_client.start() then fails with server_error
   because the stale SHM conflicts.

3. start_jack() returned True for a dead jackd — process exists but
   JackServer::Open failed with -1 (bad period/rate for device).
   No SHM socket was created, but the process check passed.

Fixes:
- _ensure_singleton(): kill any other main.py process at boot
- start_jack(): verify jack_default_0_0 SHM socket exists before
  returning True; kill zombie process if socket is missing
- Profile handler: clean SHM right before jack_client.start()
- JackAudioClient.start(): retry once with SHM cleanup on
  server_error (0x21)

main.py

@@ -55,6 +55,28 @@ from src.system.config import DEFAULT_CONFIG_PATH, load_config, save_config, DEF
 _AUTH_PIN_SEEN = False
 
 
+def _ensure_singleton() -> None:
+    """Kill any other main.py processes (from stale service restarts)."""
+    import subprocess, os
+    my_pid = os.getpid()
+    try:
+        r = subprocess.run(
+            ["pgrep", "-f", "python3.*main.py"],
+            capture_output=True, text=True, timeout=3,
+        )
+        if r.returncode == 0:
+            for pid_str in r.stdout.strip().split():
+                pid = int(pid_str)
+                if pid != my_pid:
+                    try:
+                        os.kill(pid, 9)
+                        logger.info("Killed stale pedal process PID %d", pid)
+                    except (OSError, ProcessLookupError):
+                        pass
+    except Exception:
+        pass
+
+
 def _ensure_auth_pin(config: dict, config_path: Path) -> str:
     """Generate a random 6-digit auth PIN on first boot and persist it to config.
 
@@ -790,6 +812,9 @@ def main() -> int:
 
     Returns exit code 0 on clean shutdown, 1 on boot failure.
     """
+    # Kill any stale pedal processes before doing anything else
+    _ensure_singleton()
+
     # Lock process memory early to prevent page faults in RT callback
     try:
         import ctypes

src/system/audio.py

@@ -288,10 +288,23 @@ class AudioSystem:
             logger.info("JACK disabled in config")
             return False
 
-        # Already running?
+        # Already running? Check that the running JACK matches our
+        # desired config — a zombie jackd from another process can
+        # satisfy _jack_is_running() but have wrong settings.
         if _jack_is_running():
-            logger.info("JACK already running")
-            return True
+            # Verify via /dev/shm/jack_default_0_0 socket that the
+            # running JACK is actually accepting connections.
+            if Path("/dev/shm/jack_default_0_0").is_socket():
+                logger.info("JACK already running and operational — reusing")
+                return True
+            # Process exists but no socket — likely a zombie that
+            # spawned but failed to init audio. Kill it first.
+            logger.warning("JACK process exists but not operational — killing")
+            try:
+                subprocess.run(["killall", "-9", "jackd"], capture_output=True, timeout=3)
+                time.sleep(0.5)
+            except Exception:
+                pass
 
         profile = self.config.latency_profile
         jack_env = os.environ.copy()