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/core/src/hooks/spatial-grid/spatial-grid-sync.ts

@@ -124,7 +124,7 @@ function arraysEqual(a: number[], b: number[]): boolean {
 }
 
 /**
- * Mark all floor items and walls that overlap a slab polygon as dirty.
+ * Mark all floor items, walls, and stairs that may be affected by a slab change as dirty.
  */
 function markNodesOverlappingSlab(
   slab: SlabNode,
@@ -157,6 +157,9 @@ function markNodesOverlappingSlab(
       if (wallOverlapsPolygon(wall.start, wall.end, slab.polygon)) {
         markDirty(node.id)
       }
+    } else if (node.type === 'stair') {
+      if (resolveLevelId(node, nodes) !== slabLevelId) continue
+      markDirty(node.id)
     }
   }
 }