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/view/render.rs

@@ -483,10 +483,28 @@ impl Scene {
         } else {
             self.paper_bg_color
         };
-        (adapt_to_bg(color, bg), pl, pat, lw, aci)
+        // Objects on a locked layer are dimmed toward the background so they
+        // read as "not editable" (they stay visible and snappable).
+        let adapted = adapt_to_bg(color, bg);
+        let final_color = if layer_locked(&self.document, e) {
+            crate::scene::cache::block_cache::fade_toward_bg(adapted, bg)
+        } else {
+            adapted
+        };
+        (final_color, pl, pat, lw, aci)
     }
 }
 
+/// Whether an entity sits on a locked layer (via the document's layer table).
+/// Document-only so it is safe from the parallel tessellation path.
+pub(in crate::scene) fn layer_locked(document: &CadDocument, e: &EntityType) -> bool {
+    document
+        .layers
+        .get(&e.common().layer)
+        .map(|l| l.is_locked())
+        .unwrap_or(false)
+}
+
 // ── Document-only render-style helpers (no &self, safe to call from parallel contexts) ──
 
 /// Resolves the effective linetype name for an entity, falling back to the