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>

collect.py

@@ -131,7 +131,6 @@ def run(args: argparse.Namespace) -> None:
                     save_store(out, store, args.max_records)
                     if args.diff and last_broadcast:
                         payload = jay_diff_full(last_broadcast, snapshot, combine_upd_add=True)
-                        payload["_diff"] = True
                     else:
                         payload = snapshot
                     network.broadcast(payload, push_clients, push_lock)