Phase 5 batch kind: item migrates to registry (always-on)

Item is the first kind to use the `def.renderer` escape hatch (custom
React component with `useGLTF` + drei + interactive widgets) — not
expressible as a pure `def.geometry`. Catalog-backed + multi-host
(free / wall / wall-side / ceiling).

Files added (packages/nodes/src/item/):
 - schema.ts: re-exports ItemNode from core.
 - parametrics.ts: empty groups[]. Item parametrics come from the
   asset's catalog-defined interactive controls (toggles / sliders /
   temperature) — too dynamic for the auto-inspector at Stage A.
   Legacy ItemPanel renders the catalog-driven controls; Phase 5
   Stage E will likely use parametrics.customPanel.
 - definition.ts: capabilities (no `movable` — item move is bespoke
   MoveItemContent that handles attachTo transitions floor↔wall↔
   ceiling mid-drag; capability-driven dispatch keeps legacy mover),
   parametrics, renderer (wrap-export of ItemRenderer), system
   bundling ItemSystem + ItemLightSystem, toolHints matching the
   user's screenshot (Place item / R rotate ccw / T rotate cw /
   Shift free place / Esc cancel). defaults() casts an object
   literal with a stub asset since asset is required by the schema;
   createNode re-parses through ItemNode at runtime.
 - renderer.tsx: wrap-export of legacy ItemRenderer (~280 lines with
   useGLTF + interactive widgets — too much to duplicate at Stage A).
 - system.tsx: bundles ItemSystem + ItemLightSystem.
 - index.ts: barrel.

Files changed:
 - packages/viewer/src/index.ts: new public exports for ItemRenderer,
   ItemSystem, ItemLightSystem.
 - packages/nodes/src/index.ts: appends itemDefinition.
 - packages/editor/src/components/ui/panels/item-panel.tsx: panel
   slider-drag fix recipe applied (nodeRef pattern, drop subscribed
   updateNode dep, drop node from useCallback deps). Item panel has
   scale + position + rotation sliders all subject to the cascade.

Item is the registry's stress test for `def.renderer` escape hatch.
GLB loading via useGLTF + drei works as-is; nothing in the registry
forces a pure-geometry shape on kinds that don't fit.

Phase 5 progress: shelf ✅ spawn ✅ wall ✅ fence ✅ slab ✅ ceiling ✅
door ✅ window ✅ item ✅. Nine kinds on the registry. Stair / roof /
zone / containers remain.

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

packages/viewer/src/index.ts

@@ -12,6 +12,7 @@ export { CeilingRenderer } from './components/renderers/ceiling/ceiling-renderer
 // Each kind has a geometry system + animation system to mount via
 // RegisteredSystems when the kind is registry-driven.
 export { DoorRenderer } from './components/renderers/door/door-renderer'
+export { ItemRenderer } from './components/renderers/item/item-renderer'
 export { NodeRenderer } from './components/renderers/node-renderer'
 export { WindowRenderer } from './components/renderers/window/window-renderer'
 export { default as Viewer } from './components/viewer'
@@ -50,6 +51,11 @@ export { DoorSystem } from './systems/door/door-system'
 // alongside the legacy fence mount point.
 export { FenceSystem } from './systems/fence/fence-system'
 export { InteractiveSystem } from './systems/interactive/interactive-system'
+// Item systems for the registry-driven item definition. ItemSystem
+// applies attachTo-driven transforms each frame; ItemLightSystem
+// manages item-mounted light sources.
+export { ItemSystem } from './systems/item/item-system'
+export { ItemLightSystem } from './systems/item-light/item-light-system'
 export { snapLevelsToTruePositions } from './systems/level/level-utils'
 export { getRoofMaterialArray } from './systems/roof/roof-materials'
 // Slab system follows the wall + fence re-export pattern — composed into