perf(layers): toggle layer visibility without re-tessellating

Turning a 141k-entity layer off/on took 1.5-5 s; it now takes ~0.6-0.9 s.

- Off/freeze toggles go through invalidate_layer_visibility: entities on
  the layer keep their tessellation memo (visibility gates them before
  tessellation); only block users re-expand. No mesh recolor.
- The resident patch reuses a surviving memo entry instead of
  re-tessellating, guarded by the same display-context hash as the full
  build.
- The patch looked up `layout.order` linearly per re-shown entity, which
  made turning a big layer back on quadratic.
- recolor_meshes_for_handles resolved materials for handles with no mesh.
- Wire arena: a hidden slab keeps its instances and sets a `hidden` flag
  in its consts (collapsed in the vertex shader); an unchanged re-show only
  clears the flag. Const writes are batched into merged spans.
- The `layer_visible` automation action indexed the document's layer
  table instead of the sortable panel rows the Layer* messages use.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

src/app/update/mod.rs

@@ -3338,7 +3338,7 @@ impl OpenCADStudio {
                                 pl.visible = on;
                             }
                         }
-                        self.tabs[i].scene.invalidate_layer_dependencies(&targets);
+                        self.tabs[i].scene.invalidate_layer_visibility(&targets);
                         self.tabs[i].dirty = true;
                         self.commit_layer_undo(i, undo);
                         self.command_line.push_output(
@@ -3426,7 +3426,7 @@ impl OpenCADStudio {
                                 pl.frozen = frozen;
                             }
                         }
-                        self.tabs[i].scene.invalidate_layer_dependencies(&targets);
+                        self.tabs[i].scene.invalidate_layer_visibility(&targets);
                         self.tabs[i].dirty = true;
                         self.commit_layer_undo(i, undo);
                         self.command_line.push_output(