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 <noreply@anthropic.com>

src/app/view/mod.rs

@@ -389,6 +389,11 @@ impl OpenCADStudio {
                 _ => (None, None),
             };
 
+            let hover_locked = tab
+                .scene
+                .hover_highlight
+                .map(|h| tab.scene.is_layer_locked(h))
+                .unwrap_or(false);
             crate::ui::overlay::selection_overlay(
                 sel,
                 snap_info,
@@ -402,6 +407,7 @@ impl OpenCADStudio {
                 pane_drop_rect,
                 tab.pan_mode,
                 self.ribbon.open_dropdown.is_some(),
+                hover_locked,
             )
         };