fix: mark layout parts dirty after bulk lead-in assignment

Parts were not redrawn after AssignLeadIns because their LayoutPart
graphics paths were stale.

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

OpenNest/Forms/EditNestForm.cs

@@ -741,6 +741,9 @@ namespace OpenNest.Forms
             };
             assigner.Assign(plate);
 
+            foreach (var lp in PlateView.Parts)
+                lp.IsDirty = true;
+
             PlateView.Invalidate();
         }