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/editor/src/components/editor/selection-manager.tsx

@@ -1059,13 +1059,7 @@ export const SelectionManager = () => {
       // before any of the legacy roof / stair / single-surface arms
       // below run.
 
-      if (
-        node.type === 'fence' ||
-        node.type === 'column' ||
-        node.type === 'slab' ||
-        node.type === 'ceiling' ||
-        node.type === 'shelf'
-      ) {
+      if (node.type === 'fence' || node.type === 'column' || node.type === 'shelf') {
         const compatible = paintEnabled
 
         return {

packages/editor/src/lib/material-paint.ts

@@ -382,8 +382,6 @@ export function resolveActivePaintMaterialFromSelection(params: {
   if (
     (selectedNode.type === 'fence' ||
       selectedNode.type === 'column' ||
-      selectedNode.type === 'slab' ||
-      selectedNode.type === 'ceiling' ||
       selectedNode.type === 'shelf') &&
     selectedMaterialTarget.role === 'surface'
   ) {