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/core/src/services/alignment-anchors.ts

@@ -74,7 +74,10 @@ function floorFootprint(
 ): { dimensions: [number, number, number]; rotation: [number, number, number] } | null {
   const capabilities = nodeRegistry.get(node.type)?.capabilities
   const floorPlaced = capabilities?.floorPlaced
-  if (floorPlaced) {
+  // `footprint` is optional now that floor-placed kinds may instead declare
+  // composite `footprints` (e.g. stairs); those have no single centred box
+  // here, so fall through to `alignmentFootprint`.
+  if (floorPlaced?.footprint) {
     if (floorPlaced.applies && !floorPlaced.applies(node)) return null
     return floorPlaced.footprint(node)
   }