perf(scene): keep block cache across non-block edits

The block-definition tessellation cache (BlockCache::build — every block
defn) was keyed on geometry_epoch, so every edit rebuilt all block defns.
On a block-heavy drawing that turned the ~30ms visible-wire re-tess into a
~400ms spike at line-commit, grip first-move and grip release.

Key the block cache on a separate block_epoch. bump_geometry bumps both
geometry_epoch and block_epoch (safe default — xref, block create,
explode, REFCLOSE, undo, open all keep rebuilding blocks). Operations that
provably can't change a block definition now call bump_geometry_no_blocks,
which bumps only geometry_epoch so the visible wires re-tessellate against
the reused block cache:

- add_entity for a non-Insert/Block entity (line, arc, text, hatch, …) —
  scene.add_entity only ever routes to model/paper space, never block
  content;
- grip drag hide / commit / Esc-cancel — moving a top-level entity or an
  insert never edits a block defn.

Line-commit and grip start/end drop from ~400ms to the ~30ms baseline.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/update.rs

@@ -1325,7 +1325,7 @@ impl OpenCADStudio {
                         }
                         self.tabs[i].scene.hidden.remove(&h);
                         self.tabs[i].scene.clear_preview_wire();
-                        self.tabs[i].scene.bump_geometry();
+                        self.tabs[i].scene.bump_geometry_no_blocks();
                         self.refresh_selected_grips();
                     }
                     self.tabs[self.active_tab].snap_result = None;
@@ -2127,7 +2127,9 @@ impl OpenCADStudio {
                         self.grip_original =
                             self.tabs[i].scene.document.get_entity(grip.handle).cloned();
                         self.tabs[i].scene.hidden.insert(grip.handle);
-                        self.tabs[i].scene.bump_geometry();
+                        // Grip drag never changes a block definition — keep the
+                        // block cache so the hide doesn't re-tessellate blocks.
+                        self.tabs[i].scene.bump_geometry_no_blocks();
                         self.grip_preview_handle = Some(grip.handle);
                     }
 
@@ -2650,7 +2652,7 @@ impl OpenCADStudio {
                         self.grip_original = None;
                         self.tabs[i].scene.hidden.remove(&h);
                         self.tabs[i].scene.clear_preview_wire();
-                        self.tabs[i].scene.bump_geometry();
+                        self.tabs[i].scene.bump_geometry_no_blocks();
                     }
                     // Placement confirmed — keep the just-added leader.
                     self.grip_add_provisional = None;