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

ubx/messages/nav.py

@@ -34,11 +34,13 @@ class Sig(UbxMsg):
 		remain = len(data) - offset
 		group_size = 16
 		n = offset
-		while remain:
+		while remain >= group_size:
 			signal = cls.Group1.from_bytes(data[n:n + group_size])
 			n += group_size
 			remain -= group_size
 			obj.signals.append(signal)
+		if remain:
+			print(f"Sig.from_bytes: {remain} trailing bytes unparsed (payload size mismatch)")
 		return obj.__dict__
 
 class Sat(UbxMsg):
@@ -70,9 +72,11 @@ class Sat(UbxMsg):
 		remain = len(data) - offset
 		group_size = 12
 		n = offset
-		while remain:
+		while remain >= group_size:
 			satellite = cls.Group1.from_bytes(data[n:n + group_size])
 			n += group_size
 			remain -= group_size
 			obj.satellites.append(satellite)
+		if remain:
+			print(f"Sat.from_bytes: {remain} trailing bytes unparsed (payload size mismatch)")
 		return obj.__dict__

ubx/messages/rxm.py

@@ -40,9 +40,11 @@ class RawX(UbxMsg):
 		remain = len(data) - offset
 		group_size = 32
 		n = offset
-		while remain:
+		while remain >= group_size:
 			group = cls.Group1.from_bytes(data[n:n + 32])
 			n += group_size
 			remain -= group_size
 			obj.meas.append(group)
+		if remain:
+			print(f"RawX.from_bytes: {remain} trailing bytes unparsed (payload size mismatch)")
 		return obj.__dict__