Slabs now expose two paintable faces instead of one: `surface` (top, keeps
the wood floor default and the prior slot id so painted slabs are
unaffected) and `side` (vertical walls + underside, default light grey).
The merged slab buffer is split by per-triangle face normal into a top mesh
and a side mesh, each tagged with its slot id for paint resolve/preview;
legacy whole-slab material maps onto the top only. Textures-off still
collapses to the floor role.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
