feat(editor): live floor-stacking, unified handle system, slab-hole editing + interaction polish (#375)

- Live slab-stacking Y previews for all floor-placed kinds (item/shelf/spawn/column/stair) during placement + both move pathways, via a shared core resolver; canonical positions unchanged.
- Unified 3D handle system (one drag pipeline + one visual primitive) with forgiving invisible hit-areas on every handle, kept on EDITOR_LAYER so they don't poison the MRT scene pass.
- Hover + click-to-edit slab holes in 3D (manual hole -> hole editor; stair/elevator hole -> select owner); generic cross-arrow polygon-move grip; normalized handle interaction colors.
- NaN-safe node mutations + non-finite shadow-light bounds guard.
- Built on #373 (level-scoped alignment / registry slab tool); #373 owns X/Z alignment, this owns Y floor-stacking.

packages/nodes/src/slab/hole-editor.tsx

@@ -24,6 +24,7 @@ export const SlabHoleEditor: React.FC<{ slabId: SlabNode['id']; holeIndex: numbe
   const slab = slabNode?.type === 'slab' ? (slabNode as SlabNode) : null
   const holes = slab?.holes || []
   const hole = holes[holeIndex]
+  const metadata = slab?.holeMetadata?.[holeIndex]
 
   const handlePolygonChange = useCallback(
     (newPolygon: Array<[number, number]>) => {
@@ -60,7 +61,7 @@ export const SlabHoleEditor: React.FC<{ slabId: SlabNode['id']; holeIndex: numbe
     }
   }, [slabId])
 
-  if (!(slab && hole) || hole.length < 3) return null
+  if (!(slab && hole) || hole.length < 3 || metadata?.source !== 'manual') return null
 
   return (
     <PolygonEditor