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>

client.py

@@ -57,11 +57,7 @@ def main() -> None:
                     try:
                         data = json.loads(line)
                         if args.diff:
-                            if data.get("_diff"):
-                                diff = {k: v for k, v in data.items() if k != "_diff"}
-                                state = jay_merge_full(state, diff)
-                            else:
-                                state = data
+                            state = jay_merge_full(state, data)
                             display = state
                         else:
                             display = data