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>

gui_client.py

@@ -23,7 +23,7 @@ from pathlib import Path
 from PyQt5.QtCore import QObject, QTimer, Qt, pyqtSignal
 from PyQt5.QtGui import QColor, QFont
 from PyQt5.QtWidgets import (
-    QApplication, QComboBox, QFormLayout, QGridLayout, QGroupBox,
+    QApplication, QCheckBox, QComboBox, QFormLayout, QGridLayout, QGroupBox,
     QHBoxLayout, QLabel, QLineEdit, QMainWindow, QPushButton, QScrollArea,
     QSizePolicy, QSpinBox, QStatusBar, QTabWidget, QTreeWidget, QTreeWidgetItem,
     QVBoxLayout, QWidget,
@@ -31,6 +31,8 @@ from PyQt5.QtWidgets import (
 
 import pyqtgraph as pg
 
+from utils.jay_diff import jay_merge_full
+
 pg.setConfigOption("background", "#12121e")
 pg.setConfigOption("foreground", "#aaaacc")
 
@@ -114,17 +116,21 @@ class TcpReader(QObject):
 
     def __init__(self):
         super().__init__()
-        self._sock    = None
-        self._running = False
+        self._sock      = None
+        self._running   = False
+        self._diff_mode = False
+        self._state:  dict = {}
 
-    def connect_to(self, host: str, port: int) -> str | None:
+    def connect_to(self, host: str, port: int, diff_mode: bool = False) -> str | None:
         """Open connection; returns error string or None on success."""
         try:
             self._sock = socket.create_connection((host, port), timeout=5)
-            self._sock.settimeout(None)  # blocking reads after connect
+            self._sock.settimeout(None)
         except OSError as exc:
             return str(exc)
-        self._running = True
+        self._diff_mode = diff_mode
+        self._state     = {}
+        self._running   = True
         threading.Thread(target=self._read_loop, daemon=True).start()
         return None
 
@@ -150,7 +156,16 @@ class TcpReader(QObject):
                     line = line.strip()
                     if line:
                         try:
-                            self.message.emit(json.loads(line))
+                            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.message.emit(dict(self._state))
+                            else:
+                                self.message.emit(data)
                         except json.JSONDecodeError:
                             pass
         except OSError:
@@ -198,9 +213,11 @@ class ConnectorTab(QWidget):
         self._port = QSpinBox()
         self._port.setRange(1, 65535)
         self._port.setValue(9999)
+        self._diff_cb = QCheckBox("Diff mode")
         self._load_conn_settings()
         form.addRow("Host:", self._host)
         form.addRow("Port:", self._port)
+        form.addRow(self._diff_cb)
 
         btn_row = QHBoxLayout()
         self._btn = QPushButton("Connect")
@@ -223,6 +240,10 @@ class ConnectorTab(QWidget):
         else:
             self.disconnect_requested.emit()
 
+    @property
+    def diff_mode(self) -> bool:
+        return self._diff_cb.isChecked()
+
     def _save_conn_settings(self):
         try:
             _SETTINGS_PATH.parent.mkdir(parents=True, exist_ok=True)
@@ -231,8 +252,9 @@ class ConnectorTab(QWidget):
                 data = json.loads(_SETTINGS_PATH.read_text())
             except (OSError, json.JSONDecodeError):
                 pass
-            data["host"] = self._host.text()
-            data["port"] = self._port.value()
+            data["host"]      = self._host.text()
+            data["port"]      = self._port.value()
+            data["diff_mode"] = self._diff_cb.isChecked()
             _SETTINGS_PATH.write_text(json.dumps(data, indent=2))
         except OSError:
             pass
@@ -244,6 +266,8 @@ class ConnectorTab(QWidget):
                 self._host.setText(data["host"])
             if "port" in data:
                 self._port.setValue(int(data["port"]))
+            if "diff_mode" in data:
+                self._diff_cb.setChecked(bool(data["diff_mode"]))
         except (OSError, json.JSONDecodeError, KeyError, ValueError):
             pass
 
@@ -692,7 +716,7 @@ class MainWindow(QMainWindow):
 
     def _connect(self, host: str, port: int):
         self._plot.clear_data()
-        err = self._reader.connect_to(host, port)
+        err = self._reader.connect_to(host, port, self._connector.diff_mode)
         if err:
             self._connector.set_disconnected(err)
             self._statusbar.showMessage(f"Connection failed: {err}")