Collider asset type, CollisionSystem and collision nodes

- Asset type 'collider' (box/circle) placed as child of a body object
- 'Add collider' shortcut (hierarchy context menu + inspector)
- CollisionSystem with enter/stay/exit pairs, sibling colliders excluded
- Nodes: On collision enter/stay/exit, Is touching
- Green outline in edit mode, hidden in play

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

src/core/model/Asset.ts

@@ -37,7 +37,15 @@ export interface BehaviorAsset extends AssetBase {
   graph: Graph;
 }
 
-export type Asset = SpriteAsset | CameraAsset | SceneAsset | BehaviorAsset;
+/**
+ * A collider is a shape (box or circle) used for collision detection. Placed as a child of
+ * the object it belongs to (its "body"); its position is the offset from that body.
+ */
+export interface ColliderAsset extends AssetBase {
+  type: 'collider';
+}
+
+export type Asset = SpriteAsset | CameraAsset | SceneAsset | BehaviorAsset | ColliderAsset;
 
 export function isSceneAsset(asset: Asset): asset is SceneAsset {
   return asset.type === 'scene';

src/core/model/AssetTypes.ts

@@ -1,5 +1,5 @@
 /** All asset types. Scenes are assets too, but cannot be placed in scenes. */
-export const ASSET_TYPES = ['scene', 'camera', 'sprite', 'behavior'] as const;
+export const ASSET_TYPES = ['scene', 'camera', 'sprite', 'behavior', 'collider'] as const;
 
 export type AssetType = (typeof ASSET_TYPES)[number];
 
@@ -19,5 +19,5 @@ export function isAssetType(value: unknown): value is AssetType {
  * They exist in the scene for their position, not for their looks.
  */
 export function isSymbolType(type: AssetType): boolean {
-  return type === 'camera' || type === 'behavior';
+  return type === 'camera' || type === 'behavior' || type === 'collider';
 }