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 <noreply@anthropic.com>

src/app/mod.rs

@@ -231,6 +231,12 @@ pub(super) struct OpenCADStudio {
     /// being placed (follows the cursor). `(entity handle, new-arrow grip id)`.
     /// Esc before the placement click removes it again.
     grip_add_provisional: Option<(acadrust::Handle, usize)>,
+    /// Handle hidden from the base tessellation during an in-progress grip
+    /// drag. While dragging, the edited entity is excluded from the cached
+    /// wire set and shown as a cheap overlay preview instead, so each move
+    /// updates only the overlay rather than re-tessellating the whole model.
+    /// Committed (un-hidden + one re-tess) when the drag ends. `None` = idle.
+    grip_preview_handle: Option<acadrust::Handle>,
     /// Open Quick Select panel state. `None` = panel closed. Filters are
     /// applied via `Message::QSelectApply`; the panel is dismissed on
     /// Apply / Cancel / Esc / outside-click.
@@ -1272,6 +1278,7 @@ impl OpenCADStudio {
             grip_popup: None,
             grip_pending: None,
             grip_add_provisional: None,
+            grip_preview_handle: None,
             qselect: None,
             show_ucs_icon: true,
             show_viewcube: true,