feat(paint-slots): prepared-drywall finish + fence 4 slots matching build options

- Add 'concrete-drywall' (Prepared Drywall) catalog finish + 512 KTX2/webp
  runtime assets; default walls (WALL_SLOT_DEFAULT interior+exterior) and the
  roof wall/trim band to it so roofs read continuous with walls.
- fence: replace the 2-slot panel/rail split with 4 slots that match the panel's
  build options — posts / infill (showInfill) / base (grounded only) / rail —
  each its own mesh with userData.slotId; conditional slots track the build
  state. Defaults: posts/infill/base charcoal, rail wood-finewood27.

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: 'library:concrete-plate',
-  exterior: 'library:concrete-plate',
+  interior: 'library:concrete-drywall',
+  exterior: 'library:concrete-drywall',
 }
 
 export type WallSurfaceMaterialSpec = {