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 <[email protected]>

src/app/update/mod.rs

@@ -3687,8 +3687,8 @@ impl OpenCADStudio {
                 let i = self.active_tab;
                 let types = self.tabs[i].scene.entity_type_names_in_layout();
                 let f = &mut self.tabs[i].scene.selection_filter;
-                for t in types {
-                    f.insert(t.to_string());
+                for t in types.iter() {
+                    f.insert(t.clone());
                 }
                 Task::none()
             }

src/app/view/mod.rs

@@ -1948,9 +1948,8 @@ impl OpenCADStudio {
                         selection_types: tab
                             .scene
                             .entity_type_names_in_layout()
-                            .into_iter()
-                            .map(|name| name.to_string())
-                            .collect(),
+                            .as_ref()
+                            .clone(),
                         selection_filter: &tab.scene.selection_filter,
                         tooltip_hidden: self.status_menu_tooltip_hidden,
                     };