feat(export): baked GLB export with identity, clips, cutout fix (phases 0-1)

Promote the client GLB export into the baked-artifact format from
plans/editor-baked-glb-export.md (phases 0 and 1).

- NodeMaterial -> classic MeshStandardMaterial conversion at export. The
  viewer's MeshStandard/LambertNodeMaterial set isNodeMaterial, not
  isMeshStandardMaterial, so GLTFExporter would otherwise drop every
  surface to a blank default. KTX2 (compressed) maps are decompressed via
  WebGPUTextureUtils so the exporter can embed them (PNG for now; KTX2
  re-encode is deferred to the phase-3 bake worker).
- Identity stamping from sceneRegistry: node.name = pascalId and
  extras = { pascalId, kind, label?, openable?, clips? }; all other
  userData stripped so editor/runtime ephemera never reach glTF extras.
- Door/window open clips baked from the build-once + pose-at-t primitives
  (door pascalSwingLeaf marker, window poseWindowMovingParts). Clips named
  by label ("Door 1: open"), carry extras.loop = false (consumers play
  once and hold; dumb glTF players still loop).
- Cutout fix: door/window selection hitboxes hide via material.visible,
  which onlyVisible misses, so the hitbox box plugged the wall opening.
  Non-renderable container meshes now keep their node but lose geometry;
  childless ones are removed.
- Editor-overlay stripping mirrors the thumbnail capture: emit
  thumbnail:before/after-capture so scene-layer affordances (handles,
  ceiling/site brackets) self-hide, and drop anything off SCENE_LAYER
  (gizmos, grid, zone fills).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

packages/viewer/src/index.ts

@@ -157,6 +157,9 @@ export { getVisibleWallMaterials } from './systems/wall/wall-materials'
 // 800+ lines of CSG / mitering logic during Phase 3. These exports are
 // removed in Phase 6 when the legacy mount points are deleted.
 export { WallSystem } from './systems/wall/wall-system'
-export { WindowAnimationSystem } from './systems/window/window-animation-system'
+export {
+  poseWindowMovingParts,
+  WindowAnimationSystem,
+} from './systems/window/window-animation-system'
 export { buildWindowPreviewMesh, WindowSystem } from './systems/window/window-system'
 export { ZoneSystem } from './systems/zone/zone-system'