Remove _diff marker: use self-describing jay_diff protocol

Server (network.py): send every record in diff mode starting from
prev={}, including the first — fixes history drop on new client connect.

Collect (collect.py): remove payload["_diff"] = True from live broadcast.

GUI client (gui_client.py): always call jay_merge_full in diff mode;
drop the _diff key check that gated reconstruction.

CLI client (client.py): fix `args.diff or True` guard that made --diff
irrelevant.

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

gui_client.py

@@ -159,11 +159,7 @@ class TcpReader(QObject):
                         try:
                             data = json.loads(line)
                             if self._diff_mode:
-                                if data.get("_diff"):
-                                    diff = {k: v for k, v in data.items() if k != "_diff"}
-                                    self._state = jay_merge_full(self._state, diff)
-                                else:
-                                    self._state = data
+                                self._state = jay_merge_full(self._state, data)
                                 self.message.emit(copy.deepcopy(self._state))
                             else:
                                 self.message.emit(data)