fix: sync stair elevations with floor slabs (#216)

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

packages/viewer/src/components/renderers/stair/stair-renderer.tsx

@@ -24,7 +24,8 @@ export const StairRenderer = ({ node }: { node: StairNode }) => {
 
   return (
     <group
-      position={node.position}
+      position-x={node.position[0]}
+      position-z={node.position[2]}
       ref={ref}
       rotation-y={node.rotation}
       visible={node.visible}