harden UBX parsing/dispatch and report peer-side disconnects in GUI

A struct size-mismatch or any other parse error in a listener could
propagate uncaught out of the socket-reading thread and silently kill
it. Catch broadly in the event loop and per-listener in call_listener,
and make the NAV-SAT/NAV-SIG/RXM-RAWX group parsers tolerate a
payload length that isn't an exact multiple of the group size instead
of crashing on a short struct.unpack.

Also wire NetworkBackend's socket-loss detection through to
ReceiverManager (via a queued signal, since the callback fires from
the backend thread) so the GUI reflects a peer-initiated disconnect
instead of leaving the row stuck showing "connected". Verified against
a real ZED-X20P: it drops the TCP session on its own after ~20-28s
regardless of traffic; the GUI now marks the receiver disconnected and
frees its ID for reconnecting.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AzmCaNjDb3TAqPpTwunKTY

gui/data_model.py

@@ -93,12 +93,14 @@ class ReceiverManager(QObject):
     connection_changed = pyqtSignal(str, bool)           # rid, connected
     receiver_added     = pyqtSignal(str)
     receiver_removed   = pyqtSignal(str)
+    _connection_lost   = pyqtSignal(str)                 # internal: backend thread -> GUI thread
 
     def __init__(self):
         super().__init__()
         self._states:  dict = {}
         self._configs: dict = {}
         self._lru:     list = self._load_lru()
+        self._connection_lost.connect(self._handle_connection_lost)
 
     # ── LRU ──────────────────────────────────────────────────────────────────
 
@@ -164,7 +166,8 @@ class ReceiverManager(QObject):
 
         if config.conn_type == 'tcp':
             backend = NetworkBackend(
-                [{'name': rid, 'host': config.host, 'port': config.port}]
+                [{'name': rid, 'host': config.host, 'port': config.port}],
+                on_disconnect=lambda name, r=rid: self._connection_lost.emit(r),
             )
         elif config.conn_type == 'serial':
             backend = SerialBackend(config.serial_port, config.baud_rate)
@@ -211,6 +214,13 @@ class ReceiverManager(QObject):
         self._push_lru(config)
         self.connection_changed.emit(rid, True)
 
+    def _handle_connection_lost(self, rid: str):
+        state = self._states.get(rid)
+        if not state or not state.connected:
+            return
+        print(f"Receiver '{rid}': connection lost")
+        self.remove_receiver(rid)
+
     def disconnect_receiver(self, rid: str):
         state = self._states.get(rid)
         if not state or not state.connected: