fix: persist simplified entities back to FileListItem

Without this, simplified geometry was lost on file switch and
not included in the final GetDrawings output.

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

OpenNest/Forms/CadConverterForm.cs

@@ -411,7 +411,14 @@ namespace OpenNest.Forms
             entityView1.ZoomToFit();
             entityView1.Invalidate();
 
-            // Update entity count label
+            var item = CurrentItem;
+            if (item != null)
+            {
+                item.Entities = entities;
+                item.EntityCount = entities.Count;
+                item.Bounds = entities.GetBoundingBox();
+            }
+
             lblEntityCount.Text = $"{entities.Count} entities";
         }