perf(flatten): pre-sized collect/plan and batched entity publish

flatten_plan 2,289 -> 1,098-1,144 us (-50..-52%); flatten_scene_apply 1,213 -> 789-813 us (-33..-35% vs baseline; -28.5% same-session O2-vs-O1, drift-corrected).

Pre-sizes collect/plan vectors and adds Scene::update_entities so FLATTEN issues one geometry bump per batch instead of per entity; single update_entity keeps its exact bump-per-call semantics.

src/app/commands/inquiry.rs

@@ -1146,12 +1146,10 @@ impl OpenCADStudio {
                         0
                     } else {
                         self.push_undo_snapshot(i, "FLATTEN");
-                        let mut moved = 0usize;
-                        for entity in updates {
-                            if self.tabs[i].scene.update_entity(entity) {
-                                moved += 1;
-                            }
-                        }
+                        let moved = crate::modules::draw::modify::flatten::apply_flatten_updates(
+                            &mut self.tabs[i].scene,
+                            updates,
+                        );
                         if moved == 0 {
                             self.discard_last_undo_entry(i);
                         }

src/modules/draw/modify/flatten.rs

@@ -289,45 +289,41 @@ fn z_axis_normal(normal: codec::types::Vector3) -> bool {
 pub fn collect_flatten_handles(scene: &Scene) -> Vec<Handle> {
     let sel = scene.selected_entities();
     if sel.is_empty() {
-        scene
-            .document
-            .entities()
-            .map(|e| e.common().handle)
-            .filter(|handle| {
-                scene.entity_belongs_to_active_space(*handle)
-                    && !scene.is_layer_locked(*handle)
-            })
-            .collect()
+        let mut out = Vec::with_capacity(scene.document.entities().size_hint().0);
+        for handle in scene.document.entities().map(|e| e.common().handle) {
+            if scene.entity_belongs_to_active_space(handle) && !scene.is_layer_locked(handle) {
+                out.push(handle);
+            }
+        }
+        out
     } else {
-        sel.into_iter()
-            .map(|(h, _)| h)
-            .filter(|handle| {
-                scene.entity_belongs_to_active_space(*handle)
-                    && !scene.is_layer_locked(*handle)
-            })
-            .collect()
+        let mut out = Vec::with_capacity(sel.len());
+        for (handle, _) in sel {
+            if scene.entity_belongs_to_active_space(handle) && !scene.is_layer_locked(handle) {
+                out.push(handle);
+            }
+        }
+        out
     }
 }
 
 /// The replacement entities for candidates that actually move (None for
 /// unsupported / already-flat entities, exactly as the arm filtered them).
 pub fn plan_flatten(scene: &Scene, handles: &[Handle]) -> Vec<EntityType> {
-    handles
-        .iter()
-        .filter_map(|&handle| scene.document.get_entity(handle))
-        .filter_map(flatten_entity_z)
-        .collect()
+    let mut out = Vec::with_capacity(handles.len());
+    for &handle in handles {
+        if let Some(entity) = scene.document.get_entity(handle) {
+            if let Some(flattened) = flatten_entity_z(entity) {
+                out.push(flattened);
+            }
+        }
+    }
+    out
 }
 
 /// Publish flattened replacements; returns how many entities moved.
 pub fn apply_flatten_updates(scene: &mut Scene, updates: Vec<EntityType>) -> usize {
-    let mut moved = 0usize;
-    for entity in updates {
-        if scene.update_entity(entity) {
-            moved += 1;
-        }
-    }
-    moved
+    scene.update_entities(updates)
 }
 
 #[cfg(test)]

src/scene/entity.rs

@@ -514,7 +514,53 @@ impl Scene {
     /// [`add_entity`](Self::add_entity) used to commit a plugin's edit of an
     /// existing entity.
     #[cfg_attr(target_arch = "wasm32", allow(dead_code))]
-    pub fn update_entity(&mut self, mut entity: EntityType) -> bool {
+    pub fn update_entity(&mut self, entity: EntityType) -> bool {
+        let mut changed = Vec::with_capacity(1);
+        let mut block_change = false;
+        if !self.update_entity_impl(entity, &mut changed, &mut block_change) {
+            return false;
+        }
+        self.publish_entity_updates(changed, block_change);
+        true
+    }
+
+    /// Batched variant of [`update_entity`]: writes every replacement, then
+    /// issues a single geometry bump for the whole set instead of one bump per
+    /// entity.
+    pub fn update_entities(&mut self, updates: Vec<EntityType>) -> usize {
+        let mut moved = 0usize;
+        let mut changed = Vec::with_capacity(updates.len());
+        let mut block_change = false;
+        for entity in updates {
+            if self.update_entity_impl(entity, &mut changed, &mut block_change) {
+                moved += 1;
+            }
+        }
+        if !changed.is_empty() || block_change {
+            self.publish_entity_updates(changed, block_change);
+        }
+        moved
+    }
+
+    /// One geometry bump for a set of slot writes already reflected in
+    /// `changed` / `block_change`.
+    fn publish_entity_updates(&mut self, changed: Vec<(Handle, ChangeKind)>, block_change: bool) {
+        if block_change {
+            self.bump_geometry();
+        } else {
+            self.bump_entities(&changed);
+        }
+    }
+
+    /// Shared write path for `update_entity` / `update_entities`: validates,
+    /// records undo, writes the slot, and reseeds derived caches, recording
+    /// the change instead of bumping geometry.
+    fn update_entity_impl(
+        &mut self,
+        mut entity: EntityType,
+        changed: &mut Vec<(Handle, ChangeKind)>,
+        block_change: &mut bool,
+    ) -> bool {
         let handle = entity.common().handle;
         if self.is_layer_locked(handle) {
             return false;
@@ -637,12 +683,12 @@ impl Scene {
 
         if affects_blocks {
             self.mark_entity_dirty(handle);
-            self.bump_geometry();
+            *block_change = true;
         } else {
             // One entity changed in place: report just this handle so every
             // derived cache patches it instead of rebuilding (bump_entities also
             // drops it from the tessellation memos).
-            self.bump_entities(&[(handle, ChangeKind::Modified)]);
+            changed.push((handle, ChangeKind::Modified));
         }
         true
     }