fix: add 1.5s stabilization check in start_jack to catch flaky JACK startup

JACK can briefly become operational (jack_lsp returns ports) during
startup then crash when the ALSA device fails. The initial readiness
check catches this window and returns True prematurely. Add a 0.5s
stabilization window after first success — if JACK drops, continue
the wait loop instead of returning.

Without this, profile changes succeed in the API but leave JACK in a
zombie state: process exists with SHM infrastructure but can't serve
clients.

src/system/audio.py

@@ -353,10 +353,18 @@ class AudioSystem:
                         "JACK started: period=%d, nperiods=%d, rate=%d",
                         profile["period"], profile["nperiods"], profile["rate"],
                     )
-                    # Auto-connect ports if enabled
-                    if self.config.auto_connect:
-                        self.connect_fx_ports()
-                    return True
+                    # Stabilization check — JACK can start briefly then crash
+                    # if the ALSA device is flaky. Wait 1.5s with retries.
+                    time.sleep(0.5)
+                    if _jack_is_operational():
+                        # Auto-connect ports if enabled
+                        if self.config.auto_connect:
+                            self.connect_fx_ports()
+                        return True
+                    # JACK was running but dropped — don't return yet,
+                    # continue the wait loop for a fresh attempt
+                    logger.warning("JACK appeared briefly then became non-operational")
+                    continue
                 # JACK process exists but not ready yet — keep waiting
                 continue