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/index.ts

@@ -38,6 +38,12 @@ export {
   sceneRegistry,
   useRegistry,
 } from './hooks/scene-registry/scene-registry'
+export {
+  type FloorPlacedElevationArgs,
+  getFloorPlacedElevation,
+  getFloorPlacedFootprints,
+  getFloorStackedPosition,
+} from './hooks/spatial-grid/floor-placed-elevation'
 export { pointInPolygon, spatialGridManager } from './hooks/spatial-grid/spatial-grid-manager'
 export {
   initSpatialGridSync,
@@ -79,6 +85,12 @@ export {
   type MaterialCategory,
   toLibraryMaterialRef,
 } from './material-library'
+export type {
+  FloorPlacedFootprint,
+  FloorPlacedFootprintContext,
+  FloorPlacedFootprintResolver,
+  FloorPlacedFootprintsResolver,
+} from './registry'
 export * from './registry'
 export * from './schema'
 export * from './services'