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/dimensions.ts

@@ -0,0 +1,18 @@
+import type { ShelfNode } from './schema'
+
+function clampShelfDim(value: unknown, lo: number, hi: number, fallback: number): number {
+  const v = typeof value === 'number' && Number.isFinite(value) ? value : fallback
+  return Math.min(Math.max(v, lo), hi)
+}
+
+export function sanitizeShelfDimensions(node: ShelfNode): ShelfNode {
+  return {
+    ...node,
+    width: clampShelfDim(node.width, 0.3, 3.0, 1.2),
+    depth: clampShelfDim(node.depth, 0.1, 1.0, 0.3),
+    thickness: clampShelfDim(node.thickness, 0.01, 0.1, 0.04),
+    height: clampShelfDim(node.height, 0.05, 2.5, 0.9),
+    rows: Math.round(clampShelfDim(node.rows, 1, 8, 1)),
+    columns: Math.round(clampShelfDim(node.columns, 1, 6, 1)),
+  }
+}