fix: satisfy basedpyright on the Optional subscriber and socket

CI lint failed on reportOptionalMemberAccess: prediction_thread is None
when the GUI runs without a sample feed, and _sock is nulled by run()'s
cleanup, so both attributes are Optional to the checker even though the
flagged call sites cannot see None at runtime. Guard the slot and bind
the socket to a local asserted non-None in _recv_latest.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

src/aare/gui/main_window.py

@@ -1631,7 +1631,10 @@ class MainWindow(QMainWindow):
                 if view.isVisible():
                     view.update_pixmap(pixmap)
         finally:
-            self.prediction_thread.notify_frame_displayed()
+            # The slot is only connected once the subscriber exists, but the
+            # attribute is Optional so the guard is what the type checker needs.
+            if self.prediction_thread is not None:
+                self.prediction_thread.notify_frame_displayed()
 
     @Slot(bool)
     def _on_sample_camera_availability_changed(self, available: bool) -> None:

src/aare/gui/threads/prediction_subscriber.py

@@ -201,10 +201,14 @@ class PredictionSubscriber(QThread):
         backlog that piled up while we were busy, and the stream is always
         consumed at line rate no matter how long a frame takes to decode.
         """
-        parts = self._sock.recv_multipart()  # honours RCVTIMEO, may raise zmq.Again
+        # Bind locally: run() nulls the attribute on cleanup, so it is Optional
+        # to the type checker even though it cannot be None while run() loops.
+        sock = self._sock
+        assert sock is not None, "_recv_latest called after the socket was closed"
+        parts = sock.recv_multipart()  # honours RCVTIMEO, may raise zmq.Again
         while True:
             try:
-                parts = self._sock.recv_multipart(zmq.NOBLOCK)
+                parts = sock.recv_multipart(zmq.NOBLOCK)
             except zmq.Again:
                 return parts
             self._dropped += 1