fix(viewcube): cache UCS picker names behind an epoch

The two per-frame ViewCube blocks rebuilt the named-UCS name list (N String clones plus a Vec) every frame; now Scene::cached_ucs_names memoises it behind ucs_epoch and the picker takes the Arc, so frames cost a refcount bump. Bumped on UCS SAVE, UCS DELETE, RENAME UCS, record-API ucss patch, file-open, and undo Full-swap. Tests: cached_ucs_names_returns_same_arc_until_bump pins the hit path; cached_ucs_names_invalidated_by_bump pins rebuild with new content. Full suite: 1976 passed / 0 failed / 25 ignored.

src/app/automation.rs

@@ -648,6 +648,7 @@ impl OpenCADStudio {
                         self.tabs[i].scene.document = doc;
                         self.tabs[i].scene.bump_layout_epoch();
                         self.tabs[i].scene.bump_scale_epoch();
+                        self.tabs[i].scene.bump_ucs_epoch();
                         self.tabs[i].scene.load_named_parameters_from_document();
                         self.tabs[i]
                             .scene

src/app/commands/layerprops.rs

@@ -247,6 +247,7 @@ impl OpenCADStudio {
                                 .document
                                 .ucss
                                 .add_or_replace(ucs.clone());
+                            self.tabs[i].scene.bump_ucs_epoch();
                             self.tabs[i].active_ucs = Some(ucs);
                             active_changed = true;
                             self.tabs[i].dirty = true;
@@ -261,6 +262,7 @@ impl OpenCADStudio {
                         } else if let Some(removed) =
                             self.tabs[i].scene.document.ucss.remove(&name)
                         {
+                            self.tabs[i].scene.bump_ucs_epoch();
                             let removed_handle = removed.handle;
                             for entity in self.tabs[i].scene.document.entities_mut() {
                                 if let codec::EntityType::Viewport(viewport) = entity {

src/app/commands/styleprops.rs

@@ -2997,6 +2997,7 @@ impl OpenCADStudio {
                             self.tabs[i].scene.bump_geometry_no_blocks();
                         }
                         if type_str == "UCS" {
+                            self.tabs[i].scene.bump_ucs_epoch();
                             if let Some(active) = self.tabs[i].active_ucs.as_mut() {
                                 if active.name.eq_ignore_ascii_case(&old_name) {
                                     active.name = new_name.clone();

src/app/history.rs

@@ -855,6 +855,7 @@ impl OpenCADStudio {
                     self.tabs[i].scene.invalidate_dependency_index();
                     self.tabs[i].scene.bump_layout_epoch();
                     self.tabs[i].scene.bump_scale_epoch();
+                    self.tabs[i].scene.bump_ucs_epoch();
                 }
                 StructureSnapshot::Layers(entries) => {
                     let names: Vec<String> =

src/app/record_api.rs

@@ -1611,7 +1611,12 @@ impl OpenCADStudio {
                     "app_ids" => patch_table!(app_ids),
                     "views" => patch_table!(views),
                     "vports" => patch_table!(vports),
-                    "ucss" => patch_table!(ucss),
+                    "ucss" => {
+                        patch_table!(ucss);
+                        if changed {
+                            self.tabs[i].scene.bump_ucs_epoch();
+                        }
+                    }
                     "vx_table" => patch_table!(vx_table),
                     _ => unreachable!(),
                 }

src/app/update/file.rs

@@ -1860,6 +1860,7 @@ impl OpenCADStudio {
         self.tabs[i].scene.document = doc;
         self.tabs[i].scene.bump_layout_epoch();
         self.tabs[i].scene.bump_scale_epoch();
+        self.tabs[i].scene.bump_ucs_epoch();
         // Load parameters first so imported dimensional constraints
         // can resolve their named driving values.
         self.tabs[i].scene.load_named_parameters_from_document();

src/app/view/mod.rs

@@ -1416,14 +1416,9 @@ bg={bg_ms:.1}ms n={view_count}"
                         .as_ref()
                         .map(|u| u.name.clone())
                         .unwrap_or_default();
-                    let ucs_names: Vec<String> = tab
+                    // Epoch-memoised (#34): Arc clone per frame, rebuilt only
-                        .scene
+                    // after a UCS-table bump.
-                        .document
+                    let ucs_names = tab.scene.cached_ucs_names();
-                        .ucss
-                        .iter()
-                        .map(|u| u.name.clone())
-                        .filter(|n| !n.is_empty())
-                        .collect();
                     let picker = iced::widget::pin(iced::widget::opaque(viewcube_ucs_picker(
                         ucs_current,
                         ucs_names,
@@ -1456,14 +1451,9 @@ bg={bg_ms:.1}ms n={view_count}"
                     .as_ref()
                     .map(|u| u.name.clone())
                     .unwrap_or_default();
-                let ucs_names: Vec<String> = tab
+                // Epoch-memoised (#34): Arc clone per frame, rebuilt only
-                    .scene
+                // after a UCS-table bump.
-                    .document
+                let ucs_names = tab.scene.cached_ucs_names();
-                    .ucss
-                    .iter()
-                    .map(|u| u.name.clone())
-                    .filter(|n| !n.is_empty())
-                    .collect();
                 let picker = iced::widget::pin(iced::widget::opaque(viewcube_ucs_picker(
                     ucs_current,
                     ucs_names,

src/app/view/viewcube.rs

@@ -150,10 +150,15 @@ pub(super) fn viewcube_nav_controls<'a>(
 /// Fixed width of the UCS picker, so it can be centred under the cube.
 pub(super) const UCS_PICKER_W: f32 = 84.0;
 
-/// The WCS / named-UCS selector shown under the cube.
+/// The WCS / named-UCS selector shown under the cube. `names` is the
-pub(super) fn viewcube_ucs_picker<'a>(current: String, names: Vec<String>) -> Element<'a, Message> {
+/// epoch-memoised `Scene::cached_ucs_names` Arc (#34) — a refcount bump per
+/// frame instead of N String clones.
+pub(super) fn viewcube_ucs_picker<'a>(
+    current: String,
+    names: std::sync::Arc<[String]>,
+) -> Element<'a, Message> {
     let mut options = vec!["WCS".to_string()];
-    options.extend(names);
+    options.extend(names.iter().cloned());
     let selected = if current.is_empty() {
         "WCS".to_string()
     } else {

src/scene/mod.rs

@@ -2365,6 +2365,10 @@ pub struct Scene {
     scale_epoch: u64,
     scale_picker_cache:
         RefCell<Option<(u64, f64, Option<bool>, String, std::sync::Arc<Vec<(String, f32, f64)>>)>>,
+    /// Bump on any named-UCS table mutation (UCS SAVE / DELETE / RENAME,
+    /// record-API patch, file-open, undo Full-swap).
+    ucs_epoch: u64,
+    ucs_names_cache: RefCell<Option<(u64, std::sync::Arc<[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
@@ -2756,6 +2760,8 @@ impl Scene {
             layout_names_cache: RefCell::new(None),
             scale_epoch: 0,
             scale_picker_cache: RefCell::new(None),
+            ucs_epoch: 0,
+            ucs_names_cache: RefCell::new(None),
             dependency_index_cache: RefCell::new(None),
             associative_hatch_source_cache: RefCell::new(None),
             parametric_constraints: Vec::new(),
@@ -6578,6 +6584,32 @@ impl Scene {
         list
     }
 
+    pub fn bump_ucs_epoch(&mut self) {
+        self.ucs_epoch += 1;
+    }
+
+    /// Named-UCS names for the ViewCube picker, memoised behind `ucs_epoch`
+    /// (#34, mirrors `cached_layout_names`). Same empty-name filter the view
+    /// blocks used when building the list inline every frame.
+    pub fn cached_ucs_names(&self) -> std::sync::Arc<[String]> {
+        if let Some((epoch, names)) = self.ucs_names_cache.borrow().as_ref() {
+            if *epoch == self.ucs_epoch {
+                return std::sync::Arc::clone(names);
+            }
+        }
+        let names: std::sync::Arc<[String]> = self
+            .document
+            .ucss
+            .iter()
+            .map(|u| u.name.clone())
+            .filter(|n| !n.is_empty())
+            .collect::<Vec<_>>()
+            .into();
+        *self.ucs_names_cache.borrow_mut() =
+            Some((self.ucs_epoch, std::sync::Arc::clone(&names)));
+        names
+    }
+
     /// Wire set for the Model layout, shared by every tile.
     ///
     /// The model wire geometry is **camera-independent**, so it is tessellated
@@ -14253,6 +14285,33 @@ mod layout_cache_tests {
         );
         assert!(after.iter().any(|(n, _, _)| n == "1:50"));
     }
+
+    #[test]
+    fn cached_ucs_names_returns_same_arc_until_bump() {
+        // RED: `Scene::cached_ucs_names` does not exist yet (#34).
+        let scene = Scene::new();
+        let first = scene.cached_ucs_names();
+        let second = scene.cached_ucs_names();
+        assert!(std::sync::Arc::ptr_eq(&first, &second));
+    }
+
+    #[test]
+    fn cached_ucs_names_invalidated_by_bump() {
+        // RED: `Scene::bump_ucs_epoch` does not exist yet (#34).
+        let mut scene = Scene::new();
+        let before = scene.cached_ucs_names();
+        scene
+            .document
+            .ucss
+            .add_or_replace(codec::tables::Ucs::new("TEST_CACHED_UCS"));
+        scene.bump_ucs_epoch();
+        let after = scene.cached_ucs_names();
+        assert!(
+            !std::sync::Arc::ptr_eq(&before, &after),
+            "ucs mutation + bump must replace the cached Arc"
+        );
+        assert!(after.iter().any(|n| n == "TEST_CACHED_UCS"));
+    }
 }
 
 #[cfg(test)]