feat(viewer): GLB walkthrough in viewer, monochrome, spawn/export polish

Move the first-person walkthrough into @pascal-app/viewer (BVHEcctrl +
GlbWalkthroughController) and round out the GLB-consuming viewer:

- Walkthrough: fallback ground only on level 0 (upper floors rely on baked
  slabs), hidden spawn marker, auto pointer-lock on enter, single-Esc exit via
  pointerlockchange, force perspective on enter / restore on exit.
- GlbScene: monochrome strips baked textures and recolours meshes by surface
  role using the active theme's clay tints; spawn node hidden from render.
- glb-export: camera/label/spawn identity extras for the viewer.

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

packages/viewer/src/index.ts

@@ -13,12 +13,19 @@ 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 {
+  type BVHEcctrlApi,
+  default as BVHEcctrl,
+  type MovementInput,
+} from './components/viewer/bvh-ecctrl'
 export {
   type GlbHover,
   type GlbIdentity,
   type GlbLevel,
   GlbScene,
+  type GlbWalkthrough,
 } from './components/viewer/glb-scene'
+export { GlbWalkthroughController } from './components/viewer/glb-walkthrough-controller'
 export type { HoverStyle, HoverStyles } from './components/viewer/post-processing'
 export {
   DEFAULT_HOVER_STYLES,