perf: stop rebuilding the selection filter list on every drawn point

`view-detail` on an edit frame:

    total=105.6ms  chrome=105.4   [every other section 0.0]

and the edit latency measurement puts ~102 ms of a 192 ms drawn point in
the gap between the handler returning and the scene primitive being built
— which is that same `chrome`.

The status bar asks for `entity_type_names_in_layout` on every frame to
populate the selection-filter menu, and the cache behind it was keyed on
`geometry_epoch`. Every drawn point bumps that epoch, so every drawn point
walked all 444 937 handles a model space owns to answer a question the
line did not change: adding a line to a drawing that already has lines
leaves the set of type names exactly as it was.

The cache now carries the set alongside the list, and folds edits in from
the delta journal — the same incremental shape `entity_index` already
uses. Additions can only grow the set, so they are applied directly and
the existing `Arc` is handed back untouched when nothing new appeared. A
removal can retire a type and cannot be resolved from a handle that is
already gone, so those still fall back to the full walk.

`entity_type_name` returns a literal for every arm but one — `Extended`
borrows its class name from the entity — so the set holds owned strings,
and the hot path tests `contains` before allocating.

Tested: a second line of an existing type must hand back the same `Arc`,
a new type must still appear, and the folded answer must equal what a
rebuild from scratch produces.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/scene/mod.rs

@@ -1851,7 +1851,20 @@ pub struct Scene {
     /// (geometry_epoch, layout block). The status bar asks for this on every
     /// frame to populate the selection-filter menu, but the answer only
     /// changes when entities are added or removed.
-    layout_type_names_cache: RefCell<Option<(u64, Handle, std::sync::Arc<Vec<String>>)>>,
+    /// `(epoch, block, present type names, the list handed to the UI)`.
+    ///
+    /// The set is kept alongside the list so an edit can be folded in without
+    /// rebuilding either: drawing a line into a drawing that already has lines
+    /// does not change the answer, and the walk that produces it visits every
+    /// entity the layout owns.
+    layout_type_names_cache: RefCell<
+        Option<(
+            u64,
+            Handle,
+            std::collections::BTreeSet<String>,
+            std::sync::Arc<Vec<String>>,
+        )>,
+    >,
     /// Reverse dependencies from layer/style/block definitions to the top-level
     /// entities whose resident wire runs actually change. Kept independent from
     /// `geometry_epoch`: a layer colour toggle can reuse the index, invalidate