fix: verify JACK is operational before returning from start_jack()

start_jack() only checked _jack_is_running() (pgrep -x jackd) which
returns True even when jackd exists but failed to initialize the ALSA
device — the process starts but hangs without creating SHM segments,
making jack_lsp/jack_wait fail. The profile handler then continues as
if JACK is running, jack_client.start() fails with server_error, the
pi-multifx ports never get created, and NAM CPU drops to 0%.

Fix: also verify /dev/shm/jack_default_0_0 (the JACK control socket)
exists before declaring JACK started. This socket is only created after
the ALSA device is successfully initialized, so a zombie jackd with a
bad period/rate combination won't pass the check.

src/system/audio.py

@@ -323,11 +323,19 @@ class AudioSystem:
             logger.error("jackd not found — install jackd2")
             return False
 
-        # Wait for readiness via jack_wait
+        # Wait for readiness — verify JACK is actually operational
+        # (process exists AND SHM socket is created), not just that
+        # the jackd process spawned. jackd can start but fail to
+        # initialize the audio device, leaving a zombie process that
+        # pgrep finds but jack_lsp can't connect to.
         deadline = time.monotonic() + timeout
         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():
                     logger.info(
                         "JACK started: period=%d, nperiods=%d, rate=%d",
                         profile["period"], profile["nperiods"], profile["rate"],
@@ -336,6 +344,8 @@ class AudioSystem:
                     if self.config.auto_connect:
                         self.connect_fx_ports()
                     return True
+                # JACK process exists but SHM not ready yet — keep waiting
+                continue
 
         # Timed out — check for common issues
         poll = proc.poll()