feat(paint-slots): migrate wall + procedural kinds onto node.slots

Retire the inline material fields on every slot-model kind, moving them
onto the unified node.slots model on load so painting, edit-propagation,
and the picker behave uniformly.

Walls: slots {interior,exterior}; slot-first viewer resolution threading
sceneMaterials (content folded into the wall material hash); WallRenderer
subscribes to the scene-material palette so a scene-material edit
re-renders live; wallPaint rebuilt on createSlotPaintCapability.

Load migration generalizes legacy -> slots across slab/ceiling (surface),
fence (posts/infill/base/rail), column (shaft/base/capital/frame), shelf
(shelves/frame/back), and stair (per-role tread/side/railing). Library/
scene refs pass through; inline customs mint a deduped scene material;
legacy fields cleared. No visual change (renderers already fell back to
the legacy fields). Roof/chimney/dormer/vents intentionally stay on their
role system.

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

packages/core/src/schema/nodes/wall.ts

@@ -20,6 +20,13 @@ export const WallNode = BaseNode.extend({
   interiorMaterialPreset: z.string().optional(),
   exteriorMaterial: MaterialSchema.optional(),
   exteriorMaterialPreset: z.string().optional(),
+  // Per-slot material overrides on the unified slot model, mirroring
+  // `SlabNode.slots`. Key = slot id (`interior` / `exterior`), value = a
+  // `MaterialRef` (`library:<id>` / `scene:<id>`). Absent = the declared slot
+  // default (`WALL_SLOT_DEFAULT`). The legacy `*Material*` fields above are
+  // read only by the load migration that moves them into `slots`; delete them
+  // in a follow-up once migrated scenes are the norm.
+  slots: z.record(z.string(), z.string()).optional(),
   thickness: z.number().optional(),
   height: z.number().optional(),
   curveOffset: z.number().optional(),