perf: stop rescanning every entity to fill the selection-filter menu

Pan and even plain cursor movement over the canvas ran at ~8 fps on a
large drawing. Nothing rendered was responsible: the GPU idled at 0-8%,
encoding no scene at all changed nothing, and vertices, pixels and draw
calls could each be cut without moving the number. `view` was the frame
-- 110 ms of it against 3-6 ms on a small drawing -- and inside `view`
it was the status bar.

The status bar builds the selection-filter menu, which asks for the
distinct entity type names in the layout. That call cloned the layout's
entire handle list (1 166 357 handles, ~9 MB), then did a map lookup, a
String allocation and a BTreeSet insert for each one. About 107 ms of a
110 ms frame, to produce roughly twenty names, for a menu that is only
visible when opened.

Cache the result against geometry_epoch and the layout block, since it
only changes when entities are added or removed, and make the scan
itself borrow the handle list rather than clone it and collect &str,
allocating only for the names it returns.

The immediate-mode UI rebuilds every widget on every mouse move, which is
fine as long as nothing in that path is O(document). This was.

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

src/scene/mod.rs

@@ -1715,6 +1715,11 @@ pub struct Scene {
     /// Reverse map: entity_handle → block_record_handle, built from entity_handles lists.
     /// Keyed by geometry_epoch. Eliminates the O(B) fallback scan in belongs_to_visible_block.
     entity_block_map_cache: RefCell<Option<(u64, HashMap<Handle, Handle>)>>,
+    /// Distinct entity type names present in a layout, keyed by
+    /// (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>>)>>,
     /// 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
@@ -1950,6 +1955,7 @@ impl Scene {
             annotation_affects_wires: std::cell::Cell::new(None),
             model_extents_cache: RefCell::new(None),
             entity_block_map_cache: RefCell::new(None),
+            layout_type_names_cache: RefCell::new(None),
             dependency_index_cache: RefCell::new(None),
             associative_hatch_source_cache: RefCell::new(None),
             block_defn_cache: RefCell::new(HashMap::default()),