perf(clipboard): keep large copy/paste responsive

Copying and pasting a whole drawing froze the app. Three synchronous,
main-thread hot spots scaled with the full document rather than the
selection:

- Copy centroid did a linear document scan per selected handle
  (wire_models_for / mirror_preview_parts), making copy O(H*N). Use the
  document's O(1) handle index instead.
- The paste ghost translated every point of every clipboard wire on each
  mouse move — O(points) per frame, which stalls placement of a huge
  selection. Above a point budget, rubber-band a single bounding-box
  outline instead (O(1) per frame).
- finalize_paste re-tessellated every solid in the document on each
  paste. add_entity already tessellates pasted solids, so switch to the
  incremental populate_missing_meshes_from_document.

src/app/command_driver.rs

@@ -2080,7 +2080,12 @@ impl OpenCADStudio {
             })
             .collect();
         self.merge_clipboard_ext_objects(i, &by_index);
-        self.tabs[i].scene.populate_meshes_from_document();
+        // Incremental: `add_entity` already tessellated every pasted top-level
+        // solid, and existing document solids are still cached — so only newly
+        // introduced block-definition solids need building. The full rebuild
+        // would clear and re-tessellate the entire document (every solid in the
+        // drawing) on each paste, which is what made a large paste stall.
+        self.tabs[i].scene.populate_missing_meshes_from_document();
         by_index
     }