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/registry/handles.ts

@@ -346,8 +346,10 @@ export type TranslateHandle<N = any> = {
    * `[alongX, alongOther]` — `alongOther` is Z for the 'horizontal' plane and
    * Y for 'node-normal'. Used to align the node's edges to the grid (rotation-
    * aware: swap the pair at 90°). Omit / return null for free movement.
+   * `sceneApi` is supplied for composite nodes whose footprint depends on
+   * children, such as straight stairs.
    */
-  snapExtents?: (node: N) => readonly [number, number] | null
+  snapExtents?: (node: N, sceneApi: SceneApi) => readonly [number, number] | null
   portal?: HandlePortal
 }
 

packages/core/src/registry/index.ts

@@ -57,6 +57,11 @@ export type {
   CuttableConfig,
   DragAction,
   EditorCtx,
+  FloorPlacedConfig,
+  FloorPlacedFootprint,
+  FloorPlacedFootprintContext,
+  FloorPlacedFootprintResolver,
+  FloorPlacedFootprintsResolver,
   FloorplanAffordance,
   FloorplanAffordanceModifiers,
   FloorplanAffordancePoint,

packages/core/src/registry/types.ts

@@ -1235,20 +1235,40 @@ export type SelectableConfig = {
   override?: (ctx: CapabilityCtx) => SelectableConfig | null
 }
 
+export type FloorPlacedFootprint = {
+  dimensions: [number, number, number]
+  rotation: [number, number, number]
+  position?: [number, number, number]
+}
+
+export type FloorPlacedFootprintContext = {
+  nodes: Readonly<Record<AnyNodeId, AnyNode>>
+}
+
+export type FloorPlacedFootprintResolver = (
+  node: AnyNode,
+  ctx?: FloorPlacedFootprintContext,
+) => FloorPlacedFootprint
+
+export type FloorPlacedFootprintsResolver = (
+  node: AnyNode,
+  ctx?: FloorPlacedFootprintContext,
+) => readonly FloorPlacedFootprint[]
+
 /**
  * Floor-placed kinds rest directly on a level and need their Y lifted by
  * any slab the footprint overlaps. The generic `<FloorElevationSystem>`
  * computes `slabElevation + node.position[1]` and writes it onto the
- * registered mesh on every dirty mark. `footprint` returns the world-space
- * footprint the spatial-grid manager uses to find overlapping slabs;
+ * registered mesh on every dirty mark. `footprint` returns the default
+ * world-space footprint the spatial-grid manager uses to find overlapping
+ * slabs; `footprints` lets composite kinds expose multiple footprint
+ * segments, with the canonical resolver taking the max slab elevation;
  * `applies` is an optional predicate to skip nodes that share a kind but
  * are mounted off-floor (items attached to a wall / ceiling).
  */
 export type FloorPlacedConfig = {
-  footprint: (node: AnyNode) => {
-    dimensions: [number, number, number]
-    rotation: [number, number, number]
-  }
+  footprint?: FloorPlacedFootprintResolver
+  footprints?: FloorPlacedFootprintsResolver
   applies?: (node: AnyNode) => boolean
 }