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/core/src/registry/types.ts

@@ -735,6 +735,7 @@ export type Capabilities = {
   groupable?: boolean
   selectable?: SelectableConfig
   interactive?: boolean
+  floorPlaced?: FloorPlacedConfig
 }
 
 export type CapabilityCtx = { node: AnyNode }
@@ -795,6 +796,23 @@ export type SelectableConfig = {
   override?: (ctx: CapabilityCtx) => SelectableConfig | null
 }
 
+/**
+ * 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;
+ * `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]
+  }
+  applies?: (node: AnyNode) => boolean
+}
+
 // ─── Relations ───────────────────────────────────────────────────────
 
 export type Relations = {