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/slab/paint.ts

@@ -0,0 +1,19 @@
+import type { AnyNode, SlabNode } from '@pascal-app/core'
+import { createSlotPaintCapability, previewGeometrySlot } from '../shared/slot-paint'
+
+/**
+ * Slab paint on the unified slot model. A slab has one paintable surface, so
+ * every face resolves to the `surface` slot; commit writes `node.slots.surface`
+ * (a shared scene-material or `library:` ref) like the shelf.
+ */
+export const slabPaint = createSlotPaintCapability({
+  resolveRole: () => 'surface',
+  applyPreview: previewGeometrySlot,
+  legacyEffective: (node: AnyNode) => {
+    const slab = node as SlabNode
+    if (slab.materialPreset || slab.material) {
+      return { material: slab.material, materialPreset: slab.materialPreset }
+    }
+    return null
+  },
+})