Add opt-in diff mode for network transmission (--diff / Diff mode checkbox)

Server (collect.py --diff):
- History burst: first record sent full, subsequent as jay_diff_full diffs
- Live broadcast: each snapshot diffed against the previous one
- Diff payloads tagged with "_diff": true

Client (client.py --diff):
- Reconstructs full state via jay_merge_full before printing

GUI (Connector tab "Diff mode" checkbox):
- TcpReader.connect_to() accepts diff_mode; maintains _state dict
- Merges incoming diffs before emitting to Dashboard/Plot
- Checkbox state persisted to plot_settings.json

Both sides default to off; must be enabled consistently on server and client.
utils/__init__.py added so utils.jay_diff is importable as a package.

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

server/network.py

@@ -3,6 +3,8 @@ import logging
 import socket
 import threading
 
+from utils.jay_diff import jay_diff_full
+
 log = logging.getLogger(__name__)
 
 
@@ -29,8 +31,19 @@ def _accept_loop(
                     records = in_mem
 
                 try:
-                    for record in records:
-                        conn.sendall((json.dumps(record, default=str) + "\n").encode())
+                    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:]:
+                            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())
+                    else:
+                        for record in records:
+                            conn.sendall((json.dumps(record, default=str) + "\n").encode())
                     if records:
                         log.info("Sent %d record(s) to %s", len(records), addr)
                 except OSError:
@@ -64,7 +77,7 @@ def start_push_server(host: str, port: int) -> tuple:
     server_sock.listen()
     clients: list  = []
     lock           = threading.Lock()
-    store_ref: dict = {"current": None, "preloaded": []}
+    store_ref: dict = {"current": None, "preloaded": [], "diff_mode": False}
     t = threading.Thread(
         target=_accept_loop,
         args=(server_sock, clients, lock, store_ref),