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>

client.py

@@ -9,6 +9,7 @@ Usage
 -----
   python client.py
   python client.py --host 192.168.1.10 --port 9999
+  python client.py --diff   # reconstruct from diff stream
 """
 
 import argparse
@@ -16,6 +17,8 @@ import json
 import socket
 import sys
 
+from utils.jay_diff import jay_merge_full
+
 
 def main() -> None:
     p = argparse.ArgumentParser(
@@ -24,6 +27,7 @@ def main() -> None:
     )
     p.add_argument("--host", default="127.0.0.1", help="Server host (default: 127.0.0.1)")
     p.add_argument("--port", type=int, default=9999, help="Server port (default: 9999)")
+    p.add_argument("--diff", action="store_true", help="Reconstruct full snapshots from diff stream")
     args = p.parse_args()
 
     print(f"Connecting to {args.host}:{args.port} …", flush=True)
@@ -36,6 +40,7 @@ def main() -> None:
     print("Connected. Waiting for data (Ctrl-C to quit).\n", flush=True)
     buf = ""
     count = 0
+    state: dict = {}
     try:
         with sock:
             while True:
@@ -51,8 +56,17 @@ def main() -> None:
                         continue
                     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
+                            display = state
+                        else:
+                            display = data
                         count += 1
-                        print(json.dumps(data, indent=2))
+                        print(json.dumps(display, indent=2))
                         print(f"--- #{count} ", "-" * 50, flush=True)
                     except json.JSONDecodeError as e:
                         print(f"[invalid JSON] {e}: {line!r}", file=sys.stderr)