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 <noreply@anthropic.com>

src/app/control/actions.rs

@@ -1227,9 +1227,9 @@ impl OpenCADStudio {
             "redo" => Message::Redo,
             "layer_visible" | "layer_locked" | "layer_frozen" | "layer_current" => {
                 let layer = string(req, "layer")?;
+                // The Layer* messages index the panel's (sortable) row list.
                 let index = self.tabs[self.active_tab]
-                    .scene
-                    .document
+                    .layers
                     .layers
                     .iter()
                     .position(|l| l.name == layer)