shelf: v2 — cubby default, withBottom, item hosting, paintable surface

Schema v2 adds style/rows/columns/withBack/withSides/withBottom/bracketStyle
and a `children: ItemNode[]` field for item hosting. Schema-level defaults
preserve the v1 wall-shelf visual so existing scenes load unchanged; the
placement tool spreads `shelfDefinition.defaults()` for fresh shelves
(cubby 3x2 at 1m × 0.5m × 1.8m, thickness 0.05m, back/sides/bottom on).

Four style geometries (wall-shelf / bookshelf / open-rack / cubby) share
the dimensional schema. `shelfRowSurfaceYs` exposes one host surface per
row, plus the bottom-board top when `withBottom` is on for cubby /
bookshelf.

Material is a single paintable surface (same shape walls / slabs / stairs
use); `DEFAULT_SHELF_MATERIAL` aligned with `DEFAULT_WALL_MATERIAL` so
unpainted shelves read as the canonical off-white.

Preview clones each cached material before mutating `transparent / opacity`
on the ghost — without the clone the mutation leaked into the cached
`getShelfMaterial` instance every committed shelf was using, rendering
them all see-through after the first placement preview rendered.

Store hardening: `migrateNodes` patches missing `children: []` on v1
shelves, and `updateNodesAction` reparenting tolerates a missing children
array on the new parent. `MaterialTarget` enum adds `'shelf'` so paint
mode picks up the kind.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/viewer/src/lib/materials.ts

@@ -322,6 +322,7 @@ export const DEFAULT_WINDOW_MATERIAL = new THREE.MeshStandardMaterial({
 })
 export const DEFAULT_CEILING_MATERIAL = createDefaultMaterial('#f5f5dc', 0.95)
 export const DEFAULT_ROOF_MATERIAL = createDefaultMaterial('#808080', 0.85)
+export const DEFAULT_SHELF_MATERIAL = createDefaultMaterial('#ffffff', 0.9)
 export const DEFAULT_STAIR_MATERIAL = createDefaultMaterial('#ffffff', 0.9)
 
 export function disposeMaterial(material: THREE.Material): void {