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/shaders/wire_indexed.wgsl

@@ -33,7 +33,7 @@ struct WireConst {
     align_total:    f32,
     world_half_width: f32,
     is_tapered:     f32,
-    _pad2:          f32,
+    hidden:         f32,
     marker_origin_high: vec4<f32>,
     marker_origin_low: vec4<f32>,
     marker_normal_scale: vec4<f32>,
@@ -244,6 +244,12 @@ fn marker_relative(position_high: vec3<f32>, position_low: vec3<f32>, c: WireCon
     out.min_elem       = min_elem;
     out.align_end      = c.align_end * lt_scale;
     out.align_total    = c.align_total;
+    // A hidden entity's arena slab stays resident; collapse it outside the
+    // clip volume and take the tombstone discard.
+    if c.hidden != 0.0 {
+        out.clip_pos = vec4<f32>(0.0, 0.0, -2.0, 1.0);
+        out.pattern_length = -1.0;
+    }
     return out;
 }