chore: apply linting fixes and expand ruff coverage to test suite

src/pygui/backend/controllers/session_controller.py

@@ -603,6 +603,14 @@ class SessionController(QObject):
             return average_clusters(edited, getattr(self, "_active_clusters", []))
         return edited
 
+    # TODO P2.4: throttle live-frame UI updates to ~30Hz
+    #   THINKING: every decoded serial frame triggers frameUpdated/stateChanged,
+    #   which repaints WaferMapItem + trend chart; at high stream rates the GUI
+    #   thread saturates on paint, not decode. Buffer the latest frame and flush
+    #   via a 33ms QTimer (latest-wins, no queue growth). Recording must keep
+    #   writing every frame — only the UI emit is throttled, so place the timer
+    #   here rather than in StreamReader. Depends on nothing; verify with P4.3.
+    #   See docs/pending/alpha-release-polish-plan.md §2.4.
     def _emit_current(self) -> None:
         frame = self._player.current()
         if frame is None:
@@ -828,4 +836,4 @@ class SessionController(QObject):
 
     def _flush_trend(self) -> None:
         import json
-        self.trendData.emit(json.dumps(self._trend_buffer))
+        self.trendData.emit(json.dumps(self._trend_buffer))