Mark stair nodes as dirty when slabs change so stairs properly stack on
top of floor slabs. StairSystem now computes slab-driven elevation by
sampling segment centers against the spatial grid.
Changes:
- spatial-grid-sync: mark stair nodes dirty on slab changes (same level)
- stair-system: syncStairGroupElevation + getStairSlabElevation compute
Y offset from slab data; rotateXZ helper for world-space transforms
- stair-renderer: use position-x/position-z so the system controls Y
