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/scene/mod.rs

@@ -2296,6 +2296,24 @@ impl Scene {
             .unwrap_or(false)
     }
 
+    /// True when `handle`'s entity sits on a locked layer. Locked objects stay
+    /// visible and snappable but cannot be selected or modified — callers in
+    /// the pick / modify paths consult this to skip them.
+    pub fn is_layer_locked(&self, handle: Handle) -> bool {
+        self.document
+            .get_entity(handle)
+            .map(|e| e.common().layer.clone())
+            .and_then(|name| self.document.layers.get(&name).map(|l| l.is_locked()))
+            .unwrap_or(false)
+    }
+
+    /// The name of the locked layer `handle` sits on, if any (for messages).
+    pub fn locked_layer_name(&self, handle: Handle) -> Option<String> {
+        let name = self.document.get_entity(handle).map(|e| e.common().layer.clone())?;
+        let locked = self.document.layers.get(&name).map(|l| l.is_locked()).unwrap_or(false);
+        locked.then_some(name)
+    }
+
     /// Visibility test for a solid mesh entity, mirroring the 2D wire path:
     /// honour the invisible flag, the isolate/hide set, and the layer's
     /// off/frozen state.