fix: _jack_is_operational() now creates throwaway JACK client instead of jack_lsp; standard profile period=512

_jack_is_operational(): jack_lsp is unreliable on JACK 1.9.22 (returns 0
exit even when dead, and socket path varies). Instead, try creating a
real JACK.Client('_healthcheck') which is the definitive test — exactly
what jack_client.start() does. If the client connects, JACK is truly
operational.

Standard profile: period changed from 256 to 512. The NAM engine takes
2-5ms per inference at 256/48k (5.33ms window = 93% CPU). Any jitter
causes xruns (audible pops at ~1Hz). At 512/48k (10.67ms window), the
same inference takes 40-50% CPU with 5ms headroom — far more stable.

src/system/audio.py

@@ -79,7 +79,7 @@ FOCUSRITE_PROFILES: dict[str, tuple[str, int, int]] = {
 # 48 kHz /  64 frames = 1.33 ms buffer  → aggressive, may xrun on RPi 4B
 LATENCY_PROFILES: dict[str, dict] = {
     "standard": {
-        "period": 256,
+        "period": 512,
         "nperiods": 2,
         "rate": 48000,
         "rt_priority": 70,
@@ -873,20 +873,20 @@ def _jack_is_running() -> bool:
 def _jack_is_operational() -> bool:
     """Check if JACK is actually accepting client connections.
 
-    Uses ``jack_lsp`` and checks stdout is non-empty (port names).
-    ``jack_lsp`` always exits 0 on JACK 1.9.22 even when dead —
-    only stdout content distinguishes 'JACK running' from
-    'JACK inaccessible'.
+    Creates a temporary JACK client — this is the definitive test
+    since it's exactly what ``jack_client.start()`` does.  ``jack_lsp``
+    is unreliable on JACK 1.9.22 (returns 0 even when dead, and the
+    server socket path may differ between versions).
+
+    The client is immediately deactivated and closed — it's throwaway.
     """
     try:
-        result = subprocess.run(
-            ["jack_lsp"],
-            capture_output=True, timeout=3,
-        )
-        # jack_lsp exits 0 even when dead on JACK 1.9.22 —
-        # only non-empty stdout means ports were actually listed
-        return bool(result.stdout.strip())
-    except (FileNotFoundError, subprocess.TimeoutExpired):
+        import jack
+        client = jack.Client("_healthcheck")
+        client.deactivate()
+        client.close()
+        return True
+    except Exception:
         return False