perf(scene): grip drag previews the entity instead of re-tessellating

Phase 2.2 (partial). Dragging an entity's grip called scene.apply_grip on
every mouse move, which bumped geometry and re-tessellated the whole model
per move (plus an O(N) clone of every wire to exclude the edited one from
snapping). On a large drawing that's the 30ms->400ms spike, per frame, for
the duration of the drag.

apply_grip no longer bumps. The drag flow now:
- first move: hide the edited entity from the base tessellation (one
  re-tess) and record it in grip_preview_handle;
- each move: apply the grip to the doc (no bump) and show the entity as a
  one-entity overlay preview (set_preview_wires) — the base is a cache hit;
- commit on release / Esc: un-hide, drop the preview, one final re-tess.

Because the edited entity is hidden, it's already absent from
hit_test_wires, so snapping runs against the cached set directly — no
per-move clone and no self-snap. Two re-tessellations per drag instead of
one per move.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

src/scene/mod.rs

@@ -4890,7 +4890,10 @@ impl Scene {
             }
             _ => {}
         }
-        self.bump_geometry();
+        // NOTE: no `bump_geometry()` here. The grip-drag caller hides the
+        // edited entity and previews it as an overlay during the drag (so a
+        // move doesn't re-tessellate the whole model), then bumps once on
+        // commit. Any other caller must bump geometry itself.
     }
 
     // ── Hit-test convenience: wire name → Handle ──────────────────────────