perf(scene): keep draw_depth_map warm across move/edit (phase 5)

draw_depth_map rebuilt on every geometry_epoch bump — an O(document) walk (all
objects for SortEntitiesTable, all entities grouped-by-block and ranked), ~45ms
per edit on a 788k-entity drawing, and it runs on every viewport snapshot.

But draw order depends only on each block`s entity COUNT (the rank denominator),
the entities` handles, and the SortEntitiesTable — none of which a Modify (move /
rotate / scale / colour / hide) changes (owner and type are preserved, and the map
ignores visibility). Gate the cache on the journal: an all-Modified edit keeps the
map; only Add / Remove (count change) or a table change (a `full` delta) rebuilds
it. So the common move/edit now skips the walk entirely.

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

src/scene/mod.rs

@@ -2976,11 +2976,31 @@ impl Scene {
     /// entities of different types order correctly against each other.
     pub(super) fn draw_depth_map(&self) -> Arc<HashMap<u64, f32>> {
         {
+            // Draw order depends on each block's entity COUNT (the rank
+            // denominator) and the entities' handles / SortEntitiesTable — none of
+            // which a Modify (move / rotate / scale / colour / hide) touches. So an
+            // all-Modified edit keeps the map; only Add / Remove (count change) or
+            // a table change (which arrives as a `full` delta) rebuilds it.
+            let reuse = {
                 let cache = self.draw_depth_cache.borrow();
-            if let Some((epoch, ref arc)) = *cache {
+                match *cache {
-                if epoch == self.geometry_epoch {
+                    Some((epoch, ref arc))
-                    return Arc::clone(arc);
+                        if epoch == self.geometry_epoch
+                            || matches!(
+                                self.replay_since(epoch),
+                                Some(ref d) if d.iter().all(|&(_, k)| k == ChangeKind::Modified)
+                            ) =>
+                    {
+                        Some(Arc::clone(arc))
                     }
+                    _ => None,
+                }
+            };
+            if let Some(arc) = reuse {
+                if let Some((ref mut e, _)) = *self.draw_depth_cache.borrow_mut() {
+                    *e = self.geometry_epoch;
+                }
+                return arc;
             }
         }
         use acadrust::objects::ObjectType;