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

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)

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)

server/network.py

@@ -31,16 +31,12 @@ def _accept_loop(
                     records = in_mem
 
                 try:
-                    if store_ref.get("diff_mode") and len(records) > 1:
-                        lines = [json.dumps(records[0], default=str) + "\n"]
-                        prev = records[0]
-                        for record in records[1:]:
+                    if store_ref.get("diff_mode"):
+                        prev: dict = {}
+                        for record in records:
                             diff = jay_diff_full(prev, record, combine_upd_add=True)
-                            diff["_diff"] = True
-                            lines.append(json.dumps(diff, default=str) + "\n")
                             prev = record
-                        for line in lines:
-                            conn.sendall(line.encode())
+                            conn.sendall((json.dumps(diff, default=str) + "\n").encode())
                     else:
                         for record in records:
                             conn.sendall((json.dumps(record, default=str) + "\n").encode())