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
-                        .scene
-                        .document
-                        .ucss
-                        .iter()
-                        .map(|u| u.name.clone())
-                        .filter(|n| !n.is_empty())
-                        .collect();
+                    // Epoch-memoised (#34): Arc clone per frame, rebuilt only
+                    // after a UCS-table bump.
+                    let ucs_names = tab.scene.cached_ucs_names();
                     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
-                    .scene
-                    .document
-                    .ucss
-                    .iter()
-                    .map(|u| u.name.clone())
-                    .filter(|n| !n.is_empty())
-                    .collect();
+                // Epoch-memoised (#34): Arc clone per frame, rebuilt only
+                // after a UCS-table bump.
+                let ucs_names = tab.scene.cached_ucs_names();
                 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.
-pub(super) fn viewcube_ucs_picker<'a>(current: String, names: Vec<String>) -> Element<'a, Message> {
+/// The WCS / named-UCS selector shown under the cube. `names` is the
+/// 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 {