fix: restore SelectedParts after plate switch in ActionClone

SetPlate clears SelectedParts, which broke bounds drawing and rotation
for surviving actions. Add OnPlateChanged hook so ActionClone can
re-populate SelectedParts after a plate switch.

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

OpenNest/Controls/PlateView.cs

@@ -129,6 +129,8 @@ namespace OpenNest.Controls
 
             if (currentAction == null || !currentAction.SurvivesPlateChange)
                 SetAction(typeof(ActionSelect));
+            else
+                currentAction.OnPlateChanged();
         }
 
         public string Status