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>
