feat(wafer-map): add threshold legend overlay and scrollbar polish

- Add color legend (Low/In-range/High + Auto/Manual mode)
  as a floating overlay in WaferMapView
- Expose thresholdAuto on SessionController/SessionModel
  to drive the legend's Auto · / Manual · label
- Style ReadoutPanel scrollbar (always-visible, 8px thumb)
  and reserve 14px gap so cards don't overlap it

src/pygui/backend/models/session_model.py

@@ -26,6 +26,10 @@ class SessionModel:
     def set_thresholds(self, config: ThresholdConfig) -> None:
         self._config = config
 
+    @property
+    def auto(self) -> bool:
+        return self._config.auto
+
     def reset(self) -> None:
         self._stability.reset()