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/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();