fix(ui): mark LayoutParts dirty after PushSelected so paths rebuild

Moving BasePart locations via Compactor.Push bypassed LayoutPart.Offset
which sets IsDirty. Without it, graphics paths were stale until a zoom
triggered a full rebuild.

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

OpenNest/Controls/PlateView.cs

@@ -939,6 +939,7 @@ namespace OpenNest.Controls
         {
             var movingParts = SelectedParts.Select(p => p.BasePart).ToList();
             Compactor.Push(movingParts, Plate, direction);
+            SelectedParts.ForEach(p => p.IsDirty = true);
             Invalidate();
         }