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/nodes/src/ceiling/slots.ts

@@ -2,8 +2,9 @@ import type { SlotDeclaration } from '@pascal-app/core'
 
 export type CeilingSlotId = 'surface'
 
-// Visual parity with the retired DEFAULT_CEILING_MATERIAL (warm beige).
-export const CEILING_SLOT_DEFAULT_COLOR = '#f5f5dc'
+// Soft white — the default underside colour for an unpainted ceiling. (A
+// ceiling renders flat-tinted, so this is a colour, not a `library:` finish.)
+export const CEILING_SLOT_DEFAULT_COLOR = '#f2eee6'
 
 /** A ceiling exposes a single paintable underside surface. */
 export function ceilingSlots(): SlotDeclaration[] {