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/wall/definition.ts

@@ -6,6 +6,7 @@ import { wallFloorplanSiblingOverrides } from './floorplan-overrides'
 import { wallPaint } from './paint'
 import { wallParametrics } from './parametrics'
 import { WallNode } from './schema'
+import { wallSlots } from './slots'
 
 /**
  * Wall — the Phase 3 stress test of the registry-driven node model.
@@ -56,6 +57,11 @@ export const wallDefinition: NodeDefinition<typeof WallNode> = {
     // preview through this entry rather than carrying a kind-name
     // arm.
     paint: wallPaint,
+    // Declared paintable slots (interior / exterior) with their default
+    // appearance — the same `{ slotId, label, default }` contract every other
+    // paintable kind exposes. Paint still writes the legacy inline fields via
+    // `wallPaint`; migrating those into `node.slots` is a later step.
+    slots: () => wallSlots(),
   },
 
   relations: {