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>

collect.py

@@ -55,6 +55,7 @@ from pathlib import Path
 from server import log_config, network, we_connect
 from server.data_model import ALL_DOMAINS, collect_snapshot
 from server.storage_helpers import auto_out_path, load_last_24h_records, load_store, save_store
+from utils.jay_diff import jay_diff_full
 
 log = logging.getLogger(__name__)
 
@@ -92,6 +93,7 @@ def run(args: argparse.Namespace) -> None:
     out = auto_out_path(logs_dir, vin)
 
     push_clients, push_lock, push_store_ref = network.start_push_server(args.host, args.port)
+    push_store_ref["diff_mode"] = args.diff
     push_store_ref["preloaded"] = load_last_24h_records(logs_dir, vin)
     if push_store_ref["preloaded"]:
         log.info("Pre-loaded %d record(s) from the last 24 h", len(push_store_ref["preloaded"]))
@@ -99,6 +101,7 @@ def run(args: argparse.Namespace) -> None:
     current_day = datetime.now(timezone.utc).date()
     store = load_store(out, vin, args.interval)
     push_store_ref["current"] = store
+    last_broadcast: dict = {}
 
     if args.dry:
         log.info("Push server running on %s:%d — no live collection  (Ctrl-C to stop)", args.host, args.port)
@@ -126,7 +129,13 @@ def run(args: argparse.Namespace) -> None:
                     snapshot = collect_snapshot(vehicle, domains)
                     store["records"].append(snapshot)
                     save_store(out, store, args.max_records)
-                    network.broadcast(snapshot, push_clients, push_lock)
+                    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)
+                    last_broadcast = snapshot
                     log.info("Record #%d saved at %s", len(store["records"]), snapshot["ts"])
                 except KeyboardInterrupt:
                     raise
@@ -184,6 +193,11 @@ def main() -> None:
         help="Keep only the last N records per file (default: unlimited)",
     )
     p.add_argument("-v", "--verbose", action="store_true", help="Enable debug logging")
+    p.add_argument(
+        "--diff",
+        action="store_true",
+        help="Send diffs instead of full snapshots over the push server (default: off)",
+    )
     p.add_argument(
         "--dry",
         action="store_true",