gui_client: debounce plot redraws to speed up initial record import

Replace direct _redraw() calls in PlotTab.update() with a 100 ms
single-shot QTimer. During a burst of incoming records (e.g. 500
history records on connect) only the data is accumulated; a single
redraw fires 100 ms after the last record arrives instead of once
per record (~4000 pyqtgraph redraws avoided).

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

gui_client.py

@@ -20,7 +20,7 @@ from collections import deque
 from datetime import datetime
 from pathlib import Path
 
-from PyQt5.QtCore import QObject, Qt, pyqtSignal
+from PyQt5.QtCore import QObject, QTimer, Qt, pyqtSignal
 from PyQt5.QtGui import QColor, QFont
 from PyQt5.QtWidgets import (
     QApplication, QComboBox, QFormLayout, QGridLayout, QGroupBox,
@@ -382,6 +382,11 @@ class PlotTab(QWidget):
         self._series: dict[str, deque] = {}
         self._units:  dict[str, str]   = {}
 
+        self._redraw_timer = QTimer(self)
+        self._redraw_timer.setSingleShot(True)
+        self._redraw_timer.setInterval(100)
+        self._redraw_timer.timeout.connect(self._redraw)
+
         # ── 4×2 scrollable plot grid (each cell: control bar + plot) ─────────
         plot_container = QWidget()
         plot_grid = QGridLayout(plot_container)
@@ -521,6 +526,7 @@ class PlotTab(QWidget):
     # ── public API ────────────────────────────────────────────────────────────
 
     def clear_data(self):
+        self._redraw_timer.stop()
         self._series.clear()
         self._units.clear()
         self._redraw()
@@ -550,7 +556,7 @@ class PlotTab(QWidget):
         if new_keys:
             self._refresh_combos()
 
-        self._redraw()
+        self._redraw_timer.start()
 
     # ── combo refresh + saved-trace restore ──────────────────────────────────