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/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();