feat(viewer): GLB-consuming /viewer path (baked-glb-export phase 2)

Add `GlbScene` — the viewer that renders a baked GLB artifact with no
parametric scene graph, and finish the phase-1 export contract it consumes.

Viewer (packages/viewer):
- GlbScene: loads the artifact, drives a building → level → zone → node drill
  hierarchy on useViewer.selection. Room/zone picking resolves from the floor
  plane (ray ∩ floor + point-in-polygon), node picking uses a footprint test,
  so walls/ceilings/zone-helpers never skew or block selection. Level modes
  (stacked/exploded/solo), dollhouse (hide a focused floor's ceilings + roof so
  rooms are visible and pickable), reconstructed zone floor fills + gradient
  edge borders + room labels (faded, hover-brightened), baked door/window
  open clips, click-outside / empty-space deselect, and the shared outline
  post-FX. Exported as GlbScene/GlbLevel/GlbIdentity/GlbHover.
- post-processing: composite the zone-pass tint into the base scene so rooms
  show whether or not SSGI is enabled (previously only added in the SSGI branch).

Export contract (packages/editor/src/lib/glb-export.ts):
- Convert WebGPU NodeMaterials to classic glTF-standard materials; decompress
  KTX2 maps via WebGPUTextureUtils so GLTFExporter can embed them.
- Stamp name + extras identity (pascalId/kind/label/openable/clips), bake
  door/window open clips (loop:false), strip editor overlays (off-layer +
  invisible-material hitboxes) so cutouts aren't plugged.
- Fix opaque-but-flagged-transparent materials (no spurious alphaMode=BLEND)
  and BackSide → FrontSide + winding flip (glTF has no back-face-only).
- Force levels + zones visible and stamp level display names + zone polygons so
  every floor bakes and /viewer can reconstruct rooms and label the breadcrumb.

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

packages/viewer/src/index.ts

@@ -13,6 +13,12 @@ export { ErrorBoundary } from './components/error-boundary'
 // — no per-kind re-exports needed.
 export { NodeRenderer } from './components/renderers/node-renderer'
 export { default as Viewer, type ViewerHandle } from './components/viewer'
+export {
+  GlbScene,
+  type GlbHover,
+  type GlbIdentity,
+  type GlbLevel,
+} from './components/viewer/glb-scene'
 export type { HoverStyle, HoverStyles } from './components/viewer/post-processing'
 export {
   DEFAULT_HOVER_STYLES,