Fence joins the node.slots paint model with one paintable 'surface' slot
(merged post/base/rail geometry is a single mesh). Resolution order matches
slab: textures-off joinery role -> node.slots.surface ref -> legacy inline
material/preset -> declared default (library:wood-finewood27). Tags the mesh
userData.slotId='surface' and wires capabilities.slots + paint. Stops
borrowing DEFAULT_STAIR_MATERIAL.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
