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/index.ts

@@ -59,6 +59,11 @@ export { ElevatorInteractionSystem } from './systems/elevator/elevator-interacti
 // registry-driven fence definition's `def.system`. Removed in Phase 6
 // alongside the legacy fence mount point.
 export { FenceSystem, generateFenceGeometry } from './systems/fence/fence-system'
+// Generic floor-elevation system. Lifts the rendered mesh of any kind
+// whose definition declares `capabilities.floorPlaced` by the slab
+// elevation under its footprint. Replaces the per-kind elevation block
+// that used to live inside `ItemSystem`.
+export { FloorElevationSystem } from './systems/floor-elevation/floor-elevation-system'
 export { GuideSystem } from './systems/guide/guide-system'
 export { InteractiveSystem } from './systems/interactive/interactive-system'
 // Item systems for the registry-driven item definition. ItemSystem