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/core/src/schema/nodes/shelf.ts

@@ -86,6 +86,13 @@ export const ShelfNode = BaseNode.extend({
   // unchanged once `'shelf'` is added to `MaterialTarget`.
   material: MaterialSchema.optional(),
   materialPreset: z.string().optional(),
+
+  // Per-slot material overrides, mirroring `ItemNode.slots`. Key = slot id
+  // (`shelves` / `frame` / `back`, see `shelfSlots`), value = a `MaterialRef`
+  // string (`library:<id>` or `scene:<id>`). Absent slot = fall back to the
+  // legacy whole-shelf `material` / `materialPreset`, then the registry slot
+  // default. A dangling ref renders the default (never blocks).
+  slots: z.record(z.string(), z.string()).optional(),
 })
 
 export type ShelfNode = z.infer<typeof ShelfNode>