feat(paint-slots): paintable slots on the procedural shelf (phase 5)

Proves the unified (nodeId, slotId) slot contract on a procedural generator,
beyond items and walls. A shelf now exposes three paintable slots — shelves /
frame / back — painted through the same PaintCapability dispatch and the same
node.slots: Record<slotId, MaterialRef> shape items use.

Foundation (shared, reusable by future procedural kinds):
- core: SlotDeclaration type + capabilities.slots(node) registry declaration;
  GeometryContext gains `materials` so a pure builder can resolve scene:<id>
  slot refs without importing useScene.
- viewer GeometrySystem: threads the scene material library into every builder
  ctx, and re-dirties (bypassing the geometryKey skip) any geometry node that
  references a scene material when that material changes — so editing a custom
  colour propagates to every shelf using it, matching items.

Shelf:
- schema: slots: Record<string, MaterialRef> (mirrors ItemNode).
- geometry: per-slot material resolution (slot override -> legacy whole-shelf
  -> declared default colour); every mesh stamped with userData.slotId;
  DEFAULT_SHELF_MATERIAL retired (declared default gives identical off-white).
- paint.ts: PaintCapability (resolveRole from userData.slotId, scene-material
  commit for one-off colours, preview restricted to __fromGeometry meshes so
  hosted items aren't ghosted, getEffectiveMaterial incl. legacy fallback).
- definition: paint + slots capabilities; slots folded into geometryKey.

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

packages/nodes/src/shelf/slots.ts

@@ -0,0 +1,35 @@
+import type { SlotDeclaration } from '@pascal-app/core'
+import type { ShelfNode } from './schema'
+
+export type ShelfSlotId = 'shelves' | 'frame' | 'back'
+
+// Visual parity with the retired DEFAULT_SHELF_MATERIAL (off-white).
+export const SHELF_SLOT_DEFAULT_COLOR = '#ffffff'
+
+/** Map a builder mesh name to its slot id (null = not a paintable shelf part). */
+export function shelfSlotIdForMeshName(name: string): ShelfSlotId | null {
+  if (name.startsWith('shelf-board')) return 'shelves'
+  if (name === 'shelf-back') return 'back'
+  if (
+    name.startsWith('shelf-side') ||
+    name.startsWith('shelf-post') ||
+    name.startsWith('shelf-divider') ||
+    name.startsWith('shelf-bracket') ||
+    name.startsWith('shelf-brace')
+  ) {
+    return 'frame'
+  }
+  return null
+}
+
+/** Which slots a given shelf actually exposes (depends on style/flags). */
+export function shelfSlots(node: ShelfNode): SlotDeclaration[] {
+  const slots: SlotDeclaration[] = [
+    { slotId: 'shelves', label: 'Shelves', default: SHELF_SLOT_DEFAULT_COLOR },
+  ]
+  const hasFrame = !(node.style === 'wall-shelf' && node.bracketStyle === 'hidden')
+  if (hasFrame) slots.push({ slotId: 'frame', label: 'Frame', default: SHELF_SLOT_DEFAULT_COLOR })
+  const hasBack = node.style === 'cubby' || (node.style === 'bookshelf' && node.withBack)
+  if (hasBack) slots.push({ slotId: 'back', label: 'Back', default: SHELF_SLOT_DEFAULT_COLOR })
+  return slots
+}