fix: bound the sample camera SUB receive queue

The socket used library defaults, so a GUI that stalls builds its own private
FIFO of up to 1000 messages — 20 s of video at 50 Hz — plus whatever the
autotuned kernel receive buffer holds on top. PUB/SUB pipes are per-subscriber,
which is why only aareGUI lagged while the other receivers stayed current.

RCVHWM=4 makes the producer drop frames for this subscriber once it is muted,
instead of queueing them, and a fixed RCVBUF stops the kernel hiding a further
few dozen frames behind the HWM. Measured against a flooding publisher, the
buffered backlog drops from 1000 frames to 4.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01DVqbXPoeHyqrQq5Vc8EcYP

src/aare/gui/threads/prediction_subscriber.py

@@ -30,6 +30,14 @@ class PredictionSubscriber(QThread):
         self._sock = self._ctx.socket(zmq.SUB)
         self._sock.setsockopt(zmq.RCVTIMEO, 500)
         self._sock.setsockopt(zmq.LINGER, 0)
+        # Cap the backlog the socket may hold. Both must be set before connect()
+        # to take effect on the pipe. Defaults let the SUB queue grow to 1000
+        # messages (20 s of video at 50 Hz) with megabytes more hidden in the
+        # kernel buffer, which is the backlog _recv_latest would then have to
+        # throw away every frame. RCVBUF stays well above the ~2.5 MB/s the
+        # stream actually needs, so it costs no throughput.
+        self._sock.setsockopt(zmq.RCVHWM, 4)
+        self._sock.setsockopt(zmq.RCVBUF, 512 * 1024)
 
         self._emit_images = True
         self.running = True