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/shelf/tool.tsx

@@ -15,7 +15,7 @@ import {
   useAlignmentGuides,
   useScene,
 } from '@pascal-app/core'
-import { triggerSFX } from '@pascal-app/editor'
+import { getFloorStackPreviewPosition, triggerSFX } from '@pascal-app/editor'
 import { useViewer } from '@pascal-app/viewer'
 import { useEffect, useMemo, useRef } from 'react'
 import { type Group, Vector3 } from 'three'
@@ -70,16 +70,16 @@ function getLevelLocalPosition(
     const local = (event as GridEvent).localPosition
     if (local) {
       const [sx, sz] = snapPointToGrid([local[0], local[2]], GRID_STEP)
-      return [sx, local[1] ?? 0, sz]
+      return [sx, 0, sz]
     }
     const [sx, sz] = snapPointToGrid([event.position[0], event.position[2]], GRID_STEP)
-    return [sx, event.position[1], sz]
+    return [sx, 0, sz]
   }
   worldVector.set(event.position[0], event.position[1], event.position[2])
   levelObject.updateWorldMatrix(true, false)
   levelObject.worldToLocal(worldVector)
   const [sx, sz] = snapPointToGrid([worldVector.x, worldVector.z], GRID_STEP)
-  return [sx, worldVector.y, sz]
+  return [sx, 0, sz]
 }
 
 const ShelfTool = () => {
@@ -150,8 +150,15 @@ const ShelfTool = () => {
         useAlignmentGuides.getState().clear()
       }
 
-      cursorRef.current?.position.set(ax, event.localPosition[1], az)
-      lastCursorRef.current = [ax, event.localPosition[1], az]
+      const position: [number, number, number] = [ax, 0, az]
+      const visualPosition = getFloorStackPreviewPosition({
+        node: previewNode,
+        position,
+        rotation: previewNode.rotation,
+        levelId: activeLevelId,
+      })
+      cursorRef.current?.position.set(...visualPosition)
+      lastCursorRef.current = position
 
       const prev = previousSnapRef.current
       if (!prev || prev[0] !== ax || prev[1] !== az) {