feat: onroad health overlay + 2x tire path in nightrider

System health overlay:
- Lower-right 5-metric panel: LAG (controlsState.cumLagMs), DROP
  (modelV2.frameDropPerc), TEMP (deviceState.maxTempC), CPU (max core of
  deviceState.cpuUsagePercent), MEM (deviceState.memoryUsagePercent)
- Color-coded white→yellow→red by severity (LAG: 50/200ms, DROP: 5/15%,
  TEMP: 75/88°C, CPU: 75/90%, MEM: 70/85%)
- Toggle in ClearPilot → Debug → "System Health Overlay"
- New param ClearpilotShowHealthMetrics, PERSISTENT (disk, survives
  reboots), default false — re-polled every ~2s so toggle takes effect
  without process restart
- InterFont(90, Bold) to match speed limit numeric styling, 30px margin,
  40px between rows, black rounded background

Nightrider center lane path (the "tire track" polygon from
scene.track_vertices) is now drawn at 2x the width of other lines —
highlights the planned path distinctly against the otherwise stark
outline-only rendering.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

selfdrive/ui/qt/home.cc

@@ -338,6 +338,15 @@ ClearPilotPanel::ClearPilotPanel(QWidget* parent) : QFrame(parent) {
   });
   debug_panel->addItem(telemetry_toggle);
 
+  auto *health_metrics_toggle = new ToggleControl("System Health Overlay",
+    "Show controls lag, model frame drops, temperature, CPU, and memory usage "
+    "in the lower-right of the onroad UI. For diagnosing slowdown issues.", "",
+    Params().getBool("ClearpilotShowHealthMetrics"), this);
+  QObject::connect(health_metrics_toggle, &ToggleControl::toggleFlipped, [](bool on) {
+    Params().putBool("ClearpilotShowHealthMetrics", on);
+  });
+  debug_panel->addItem(health_metrics_toggle);
+
   auto *vpn_toggle = new ToggleControl("VPN",
     "Connect to vpn.hanson.xyz for remote SSH access. "
     "Disabling kills the active tunnel and stops reconnection attempts.", "",