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