feat(layers): enforce layer lock (select/modify/fade + cursor badge)

Locked-layer objects were fully editable. Now they stay visible and
snappable but are otherwise protected:

- Not selectable: the five interactive pick paths (box, lasso, single
  click, cycling, box-release) plus SELECTALL and QSELECT skip locked
  entities.
- Not modifiable (defense-in-depth even if a handle slips through):
  transform_entities, erase_entities, apply_grip and the Properties
  commit all skip locked entities.
- Faded: entities on a locked layer render dimmed toward the background
  (render_style + wire tessellation); toggling lock re-tessellates.
- Snap still works: locked entities are intentionally kept in the wire
  set (visibility_ok untouched), so object snap keeps finding them.
- Feedback: a small padlock is drawn by the crosshair when hovering a
  locked object, and clicking one prints "Object is on locked layer …".

New Scene::is_layer_locked / locked_layer_name helpers centralise the
check (mirroring layer_hidden).

Co-Authored-By: Claude Opus 4.8 <[email protected]>

src/app/commands/inquiry.rs

@@ -16,6 +16,8 @@ impl OpenCADStudio {
                     .entity_wires()
                     .iter()
                     .filter_map(|w| Scene::handle_from_wire_name(&w.name))
+                    // Objects on a locked layer aren't selectable.
+                    .filter(|&h| !self.tabs[i].scene.is_layer_locked(h))
                     .collect();
                 let count = handles.len();
                 for h in handles {