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/modify.rs

@@ -5,6 +5,11 @@ impl Scene {
     // ── Modify (transform / copy) ─────────────────────────────────────────
 
     pub fn transform_entities(&mut self, handles: &[Handle], t: &EntityTransform) {
+        // Never transform objects on a locked layer (defense-in-depth: the pick
+        // path already excludes them, but programmatic callers may not).
+        let handles: Vec<Handle> =
+            handles.iter().copied().filter(|&h| !self.is_layer_locked(h)).collect();
+        let handles = &handles[..];
         // MIRRTEXT (header.mirror_text): when false AutoCAD positions text /
         // mtext / shape by the mirror but keeps the original rotation +
         // oblique so the text stays right-reading. Capture before the
@@ -254,6 +259,10 @@ impl Scene {
     // ── Grip editing ──────────────────────────────────────────────────────
 
     pub fn apply_grip(&mut self, handle: Handle, grip_id: usize, apply: GripApply) {
+        // Objects on a locked layer can't be grip-edited.
+        if self.is_layer_locked(handle) {
+            return;
+        }
         // For Solid3D / Region / Body, record the old point_of_reference so we
         // can translate the pre-tessellated MeshModel by the same delta after
         // the grip is applied (the ACIS data itself is not modified).