Feat/stairs fence update (#226)

* feat: railing on the straight stairs and new fence

* feat: added spiral and curved stairs with bug fix for fence

* feat:fence are linked to each other ... so moving one move the other sharing the same coordinate

* fix: update stair railing logic to include front-side attachments for terminal landings

* Integrate fence rendering into the fence system

* fix: pass nodeId instead of undefined node to WallTreeNode and FenceTreeNode

TreeNode was passing `node` (undefined variable) instead of `nodeId` to
WallTreeNode and FenceTreeNode, causing a runtime ReferenceError.
Updated FenceTreeNode to accept nodeId and look up the node from the
scene store internally, consistent with all other tree node components.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* feat: update fence icon with new isometric design

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

---------

Co-authored-by: Aymeric Rabot <aymeric@pascal.app>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

packages/core/src/index.ts

@@ -4,6 +4,7 @@ export type {
   CeilingEvent,
   DoorEvent,
   EventSuffix,
+  FenceEvent,
   GridEvent,
   ItemEvent,
   LevelEvent,
@@ -44,6 +45,7 @@ export {
   useInteractive,
 } from './store/use-interactive'
 export { default as useLiveTransforms, type LiveTransform } from './store/use-live-transforms'
+export { FenceSystem } from './systems/fence/fence-system'
 export { clearSceneHistory, default as useScene } from './store/use-scene'
 export { CeilingSystem } from './systems/ceiling/ceiling-system'
 export { DoorSystem } from './systems/door/door-system'