fix: clean stale JACK SHM segments on stop_jack() and at boot

When JACK is killed forcefully (killall -9), the /dev/shm/jack_sem.*
shared memory segments and /dev/shm/jack_db-0/ directory are left
behind. The next start_jack() call finds these stale segments and
fails silently — no audio output, JACK process may appear briefly
then crash.

Fix:
- AudioSystem.stop_jack(): after killing jackd, also rm -rf
  /dev/shm/jack* to clean up stale SHM.
- main.py boot sequence: clean SHM before first start_jack(), so
  the pedal recovers from crashes that happened before this fix.

src/system/audio.py

@@ -359,6 +359,18 @@ class AudioSystem:
             subprocess.run(["killall", "-9", "jackd"], capture_output=True)
         except FileNotFoundError:
             pass
+        # Clean up stale JACK shared memory segments — these prevent a
+        # new JACK instance from starting if left behind after a crash
+        # or forceful kill.
+        try:
+            import shutil
+            for p in Path("/dev/shm").glob("jack*"):
+                if p.is_dir():
+                    shutil.rmtree(p, ignore_errors=True)
+                else:
+                    p.unlink(missing_ok=True)
+        except Exception:
+            pass
 
     def restart_jack(self, timeout: int = 10) -> bool:
         """Restart JACK server."""