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/scene/preview.rs

@@ -26,7 +26,7 @@ impl Scene {
         handles
             .iter()
             .flat_map(|h| {
-                match self.document.entities().find(|e| e.common().handle == *h) {
+                match self.document.get_entity(*h) {
                     // Hatches carry no outline in the normal wire set, but an
                     // edit preview (move / copy / array / grip-drag) needs to
                     // show the shape following the cursor. Build a live boundary
@@ -54,7 +54,7 @@ impl Scene {
         let mut plain: Vec<WireModel> = Vec::new();
         let mut texts: Vec<(WireModel, glam::DVec3)> = Vec::new();
         for h in handles {
-            let Some(e) = self.document.entities().find(|e| e.common().handle == *h) else {
+            let Some(e) = self.document.get_entity(*h) else {
                 continue;
             };
             if matches!(e, EntityType::Text(_)) {