gui_client: fix diff mode display corruption via deepcopy on emit

jay_merge_full/jay_merge_update mutates _state nested dicts in place.
The previously emitted dict(_state) was a shallow copy sharing those
nested dicts, so background-thread merges retroactively corrupted
already-emitted snapshots: timestamps were correct (strings are
immutable) but Y values all converged to the latest record's values.

Fix: emit copy.deepcopy(_state) so each snapshot is fully independent.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

gui_client.py

@@ -13,6 +13,7 @@ Plot       — time-series charts of all physical {value, unit} fields
 
 import json
 import socket
+import copy
 import sys
 import threading
 import time
@@ -163,7 +164,7 @@ class TcpReader(QObject):
                                     self._state = jay_merge_full(self._state, diff)
                                 else:
                                     self._state = data
-                                self.message.emit(dict(self._state))
+                                self.message.emit(copy.deepcopy(self._state))
                             else:
                                 self.message.emit(data)
                         except json.JSONDecodeError: