registry: generic FloorElevationSystem driven by a floorPlaced capability

Symptom: a shelf placed on a level with a raised slab underneath
visually clipped through it — `ItemSystem` lifted items onto slabs,
but shelves (and other floor-placed registry kinds) had no equivalent
path.

Fix: lift the slab-elevation logic out of `ItemSystem` into a generic
`<FloorElevationSystem>` keyed off a new `capabilities.floorPlaced`
config. Any kind that opts in declares a `footprint(node)` (dimensions
+ rotation used to query overlapping slabs) and an optional `applies`
predicate (skips items whose `asset.attachTo` is wall / ceiling).

The new system runs at frame priority 1 so its `mesh.position.y`
override lands before `ItemSystem` / `GeometrySystem` (priority 2)
clear the dirty mark. The spatial-grid sync's `markNodesOverlappingSlab`
also dropped its hardcoded `item` branch in favour of an iteration over
every registered kind that declares `floorPlaced` — so any new
floor-placed kind picks up slab-driven re-elevation automatically.

Tagged kinds:
 - `item`  — `footprint = getScaledDimensions`, `applies = !asset.attachTo`
 - `shelf` — `footprint = (w, h, d)`
 - `column` — `footprint = (w, h, d)`
 - `spawn` — `footprint = (0.6, 1.8, 0.6)` (marker)

`ItemSystem` retains only the wall-side z-offset block (`mesh.position.z =
wallThickness / 2`). The elevation block + its `getScaledDimensions` /
`resolveLevelId` / `spatialGridManager` imports moved to the generic
system.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/viewer/src/systems/item/item-system.tsx

@@ -1,10 +1,7 @@
 import {
   type AnyNodeId,
-  getScaledDimensions,
   type ItemNode,
-  resolveLevelId,
   sceneRegistry,
-  spatialGridManager,
   useScene,
   type WallNode,
 } from '@pascal-app/core'
@@ -15,6 +12,13 @@ import type * as THREE from 'three'
 // ITEM SYSTEM
 // ============================================================================
 
+/**
+ * Per-frame wall-side offset for items mounted to wall faces. The slab-
+ * elevation lift for floor items lives in the generic
+ * `<FloorElevationSystem>` and runs at priority 1 — it has already
+ * landed `mesh.position.y` by the time this system clears the dirty
+ * mark at priority 2.
+ */
 export const ItemSystem = () => {
   const dirtyNodes = useScene((state) => state.dirtyNodes)
   const clearDirty = useScene((state) => state.clearDirty)
@@ -39,20 +43,6 @@ export const ItemSystem = () => {
           const side = item.side === 'front' ? 1 : -1
           mesh.position.z = (wallThickness / 2) * side
         }
-      } else if (!item.asset.attachTo) {
-        // If parented to another item (surface placement), R3F handles positioning via the hierarchy
-        const parentNode = item.parentId ? nodes[item.parentId as AnyNodeId] : undefined
-        if (parentNode?.type !== 'item') {
-          // Floor item: elevate by slab height (using full footprint overlap)
-          const levelId = resolveLevelId(item, nodes)
-          const slabElevation = spatialGridManager.getSlabElevationForItem(
-            levelId,
-            item.position,
-            getScaledDimensions(item),
-            item.rotation,
-          )
-          mesh.position.y = slabElevation + item.position[1]
-        }
       }
 
       clearDirty(id as AnyNodeId)