feat(paint-slots): unified slot defaults + paint for slab, ceiling, wall (phase 5)

Brings slab, ceiling, and wall onto the unified slot contract the shelf
established, so each declares its paintable slots with a declarative default and
(slab/ceiling) is painted through the registry capabilities.paint dispatch.

- Shared helper packages/nodes/src/shared/slot-paint.ts: a node.slots-based
  PaintCapability factory (commit/resolve/effective generic; preview injected).
  Distinct from surface-paint.ts, which writes the legacy inline node.material.
- slab: schema slots; def.geometry resolves node.slots.surface -> legacy
  material -> declared default, tags the mesh userData.slotId; slabPaint +
  capabilities.slots. Retires DEFAULT_SLAB_MATERIAL in the slab path.
- ceiling: schema slots; material builders extracted to ceiling/materials.ts
  (shared by renderer + paint preview, built BackSide so the hover preview is
  visible from below); renderer resolves the slot; ceilingPaint + slots.
- wall: WALL_SLOT_DEFAULT in core; the viewer's getMaterialsForWall renders an
  unpainted face with its declared default instead of the themed wall role;
  capabilities.slots (interior/exterior). wallPaint's inline interior/exterior
  fields are unchanged (node.slots migration is a later step).
- selection-manager + material-paint: drop slab/ceiling from the legacy
  single-surface arms (now registry-driven).

Behavior change (intended, matches the shelf precedent + the phase-5 plan):
colored-mode UNPAINTED slab/ceiling/wall surfaces now render their fixed slot
default (#e5e5e5 / #f5f5dc / #ffffff) instead of the theme role colour. The
textures-off (monochrome) role collapse is unchanged — the escape hatch.

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

packages/core/src/schema/index.ts

@@ -154,6 +154,7 @@ export type { WallSurfaceMaterialSpec, WallSurfaceSide } from './nodes/wall'
 export {
   getEffectiveWallSurfaceMaterial,
   getWallSurfaceMaterialSignature,
+  WALL_SLOT_DEFAULT,
   WallNode,
 } from './nodes/wall'
 export { WindowNode, WindowType } from './nodes/window'

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

@@ -11,6 +11,10 @@ export const CeilingNode = BaseNode.extend({
   children: z.array(ItemNode.shape.id).default([]),
   material: MaterialSchema.optional(),
   materialPreset: z.string().optional(),
+  // Per-slot material overrides on the unified slot model, mirroring
+  // `ShelfNode.slots`. Key = slot id (`surface`), value = a `MaterialRef`
+  // (`library:<id>` / `scene:<id>`). Absent = the declared slot default.
+  slots: z.record(z.string(), z.string()).optional(),
   polygon: z.array(z.tuple([z.number(), z.number()])),
   holes: z.array(z.array(z.tuple([z.number(), z.number()]))).default([]),
   holeMetadata: z.array(SurfaceHoleMetadata).default([]),

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

@@ -9,6 +9,10 @@ export const SlabNode = BaseNode.extend({
   type: nodeType('slab'),
   material: MaterialSchema.optional(),
   materialPreset: z.string().optional(),
+  // Per-slot material overrides on the unified slot model, mirroring
+  // `ShelfNode.slots`. Key = slot id (`surface`), value = a `MaterialRef`
+  // (`library:<id>` / `scene:<id>`). Absent = the declared slot default.
+  slots: z.record(z.string(), z.string()).optional(),
   polygon: z.array(z.tuple([z.number(), z.number()])),
   holes: z.array(z.array(z.tuple([z.number(), z.number()]))).default([]),
   holeMetadata: z.array(SurfaceHoleMetadata).default([]),

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

@@ -46,6 +46,16 @@ export type WallNode = z.infer<typeof WallNode>
 
 export type WallSurfaceSide = 'interior' | 'exterior'
 
+// Declared default appearance for an unpainted wall face in colored mode —
+// visual parity with the retired DEFAULT_WALL_MATERIAL. Lives in core so the
+// slot declaration (nodes) and the material resolver (viewer) share one value.
+// 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',
+}
+
 export type WallSurfaceMaterialSpec = {
   material?: z.infer<typeof MaterialSchema>
   materialPreset?: string