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/scene/pipeline/wire_gpu.rs

@@ -118,7 +118,9 @@ pub struct WireConst {
     /// in drawing units.
     pub world_half_width: f32,
     pub is_tapered: f32,
-    pub _pad2: f32,
+    /// Non-zero hides every segment of the wire: arena slabs of hidden
+    /// entities stay resident and are collapsed by the vertex shader.
+    pub hidden: f32,
     /// Point-marker origin as a double-single pair. `marker_normal_scale.w`
     /// stores the viewport-height percentage; zero disables marker scaling.
     pub marker_origin_high: [f32; 4],
@@ -776,7 +778,7 @@ pub(crate) fn emit_wire_native(
         align_total,
         world_half_width: wire.world_width * 0.5,
         is_tapered: if is_tapered { 1.0 } else { 0.0 },
-        _pad2: 0.0,
+        hidden: 0.0,
         marker_origin_high,
         marker_origin_low,
         marker_normal_scale,