feat(paint-slots): concrete-plate finish + try defaults (wall=concrete, slab=wood plank 48, ceiling=soft white)

- material-library: add 'concrete-plate' KTX2 finish (512, fabric/leather-style
  pipeline) + editor-app texture mirror.
- viewer: shared resolveSlotDefaultMaterial(colour|library ref) so a kind's slot
  default can be a catalog finish, not just a flat colour.
- wall default -> library:concrete-plate (interior + exterior).
- slab default -> library:wood-woodplank48 (slab geometry resolves it via the
  shared helper).
- ceiling default -> soft white #f2eee6 (ceiling renders flat-tinted).

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

packages/core/src/schema/nodes/wall.ts

@@ -52,8 +52,8 @@ export type WallSurfaceSide = 'interior' | 'exterior'
 // May be a `#rrggbb` colour or a `library:<id>` ref. Textures-off still
 // collapses to the themed wall role (the escape hatch).
 export const WALL_SLOT_DEFAULT: Record<WallSurfaceSide, string> = {
-  interior: '#ffffff',
-  exterior: '#ffffff',
+  interior: 'library:concrete-plate',
+  exterior: 'library:concrete-plate',
 }
 
 export type WallSurfaceMaterialSpec = {