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/nodes/src/ceiling/definition.ts

@@ -10,8 +10,10 @@ import {
   ceilingMoveVertexAffordance,
 } from './floorplan-affordances'
 import { ceilingFloorplanMoveTarget } from './floorplan-move'
+import { ceilingPaint } from './paint'
 import { ceilingParametrics } from './parametrics'
 import { CeilingNode } from './schema'
+import { ceilingSlots } from './slots'
 
 const HEIGHT_HANDLE_OFFSET = 0.22
 const MIN_CEILING_HEIGHT = 0.5
@@ -102,6 +104,10 @@ export const ceilingDefinition: NodeDefinition<typeof CeilingNode> = {
     },
     duplicable: true,
     deletable: true,
+    // Unified slot model: one paintable underside surface with a declared
+    // default, painted through the registry `capabilities.paint` dispatch.
+    slots: () => ceilingSlots(),
+    paint: ceilingPaint,
   },
 
   relations: {