feat(tabletop): show surface bounds and lay parts flat

Add a toggleable surface plane, bounds outline, and constant-size HTML
label. Scale placement and mount anchors to world units so parts land on
the surface, and rotate parts to face up.

packages/tabletop/src/index.ts

@@ -33,6 +33,8 @@ export { resolveMountTree } from './mount.js';
 export type { MountNode, MountTree } from './mount.js';
 export { WorldSurfaceView, SurfaceNode } from './surfaces/WorldSurfaceView.js';
 export { HudSurfaceView } from './surfaces/HudSurfaceView.js';
+export { SurfaceBounds } from './surfaces/SurfaceBounds.js';
+export { SurfacePlane } from './surfaces/SurfacePlane.js';
 
 // Part placement.
 export { PartPlacement } from './placement.js';