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>

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())