fix: replace broken jack_default_0_0 socket check with jack_lsp for JACK 1.9.22 compatibility

start_jack() was checking for /dev/shm/jack_default_0_0 which never
exists on JACK 1.9.22 (used on RPi/DietPi). The check made start_jack()
always timeout and return False. Boot sequence ignored the return value
(luck), but the profile change handler checked it via 'if not ok: rollback',
causing all buffer/rate changes to fail silently every other time.

Replace with _jack_is_operational() using jack_lsp which tests actual
JACK IPC and works correctly on all JACK versions.

src/system/audio.py

@@ -291,13 +291,14 @@ class AudioSystem:
         # 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.
+        # IMPORTANT: On JACK 1.9.22 there is no /dev/shm/jack_default_0_0
+        # socket. Instead, verify via jack_lsp which tests actual IPC.
         if _jack_is_running():
-            # 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():
+            # Verify JACK is actually accepting connections
+            if _jack_is_operational():
                 logger.info("JACK already running and operational — reusing")
                 return True
-            # Process exists but no socket — likely a zombie that
+            # Process exists but can't connect — likely a zombie that
             # spawned but failed to init audio. Kill it first.
             logger.warning("JACK process exists but not operational — killing")
             try:
@@ -345,10 +346,9 @@ class AudioSystem:
         while time.monotonic() < deadline:
             time.sleep(0.3)
             if _jack_is_running():
-                # Double-check JACK is actually operational by testing
-                # the SHM socket — jack_default_0_0 is created only
-                # after the ALSA device is successfully initialized.
-                if Path("/dev/shm/jack_default_0_0").is_socket():
+                # Verify JACK is actually operational via jack_lsp
+                # (jack_default_0_0 socket doesn't exist on JACK 1.9.22)
+                if _jack_is_operational():
                     logger.info(
                         "JACK started: period=%d, nperiods=%d, rate=%d",
                         profile["period"], profile["nperiods"], profile["rate"],
@@ -357,7 +357,7 @@ class AudioSystem:
                     if self.config.auto_connect:
                         self.connect_fx_ports()
                     return True
-                # JACK process exists but SHM not ready yet — keep waiting
+                # JACK process exists but not ready yet — keep waiting
                 continue
 
         # Timed out — check for common issues
@@ -851,6 +851,27 @@ def _jack_is_running() -> bool:
         return False
 
 
+def _jack_is_operational() -> bool:
+    """Check if JACK is actually accepting client connections.
+
+    Uses ``jack_lsp`` to verify the JACK server is operational beyond
+    just having a running process. ``jack_lsp`` connects to the JACK
+    server via shared memory and lists ports — it only succeeds when
+    JACK has fully initialized the ALSA device and IPC is live.
+
+    On JACK 1.9.22 there is no ``/dev/shm/jack_default_0_0`` socket,
+    so ``jack_lsp`` is the most reliable readiness check.
+    """
+    try:
+        result = subprocess.run(
+            ["jack_lsp"],
+            capture_output=True, timeout=3,
+        )
+        return result.returncode == 0
+    except (FileNotFoundError, subprocess.TimeoutExpired):
+        return False
+
+
 # ═══════════════════════════════════════════════════════════════════
 # JACK audio client (real-time I/O with pipeline)
 # ═══════════════════════════════════════════════════════════════════