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.

main.py

@@ -130,6 +130,19 @@ class PedalApp:
             )
             self.audio_system = AudioSystem(self.audio_config)
             self.audio_system.setup_i2s(reboot_hint=True)
+
+            # Clean stale JACK SHM segments from prior crashes before starting
+            try:
+                import shutil
+                from pathlib import Path
+                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
+
             if self.audio_config.jack_enabled:
                 self.audio_system.start_jack(timeout=10)
             else: