fix(layers): close locked-layer gaps found in review

- Double-clicking a locked object no longer opens its text / attribute /
  in-place block editor.
- Clicking a locked object during a command's object-gather no longer
  counts as completing the selection (don't set selection_just_completed
  on the locked path).
- copy_entities skips locked entities.
- The attribute editor's Apply refuses to write when the block's layer was
  locked while the dialog was open.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/scene/modify.rs

@@ -209,8 +209,10 @@ impl Scene {
     }
 
     pub fn copy_entities(&mut self, handles: &[Handle], t: &EntityTransform) -> Vec<Handle> {
+        // Objects on a locked layer can't be copied (they can't be selected).
         let clones: Vec<EntityType> = handles
             .iter()
+            .filter(|&&h| !self.is_layer_locked(h))
             .filter_map(|&h| self.document.get_entity(h).cloned())
             .collect();
         let mut new_handles = Vec::with_capacity(clones.len());