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/commands/assetCommands.ts

@@ -19,6 +19,8 @@ export function buildAsset(type: AssetType, name: string, id = newId()): Asset {
       return { ...base, type: 'sprite' };
     case 'behavior':
       return { ...base, type: 'behavior', variables: [], graph: createEmptyGraph() };
+    case 'collider':
+      return { ...base, type: 'collider' };
   }
 }
 

src/core/commands/colliderCommands.ts

@@ -0,0 +1,44 @@
+import { findScene, type Project } from '../model/Project';
+import { createSceneObject } from '../model/SceneObject';
+import { resolveSceneObjectProps } from '../properties/resolve';
+import { newId } from '../ids';
+import { buildAsset, requireAsset } from './assetCommands';
+import { type Command, CommandError } from './Command';
+import { nextAssetName } from './projectCommands';
+import { requireObject } from './sceneObjectCommands';
+
+/**
+ * The one-click way to give an object a collider: creates a box collider asset sized like
+ * the object (sprites: width × height; anything else: schema defaults) and places it as a
+ * child at the object's centre. Undo removes both the placed collider and the asset.
+ */
+export class AddColliderCommand implements Command {
+  readonly label = 'Add collider';
+  readonly assetId = newId();
+  readonly objectId = newId();
+
+  constructor(private readonly sceneId: string, private readonly targetId: string) {}
+
+  execute(project: Project): void {
+    const scene = findScene(project, this.sceneId);
+    if (!scene) throw new CommandError(`Scene ${this.sceneId} not found`);
+    const target = requireObject(project, this.sceneId, this.targetId);
+    const targetAsset = requireAsset(project, target.assetId);
+    if (targetAsset.type === 'collider') throw new CommandError('A collider cannot have a collider');
+
+    const asset = buildAsset('collider', nextAssetName(project, `${target.name || targetAsset.name} collider`), this.assetId);
+    if (targetAsset.type === 'sprite') {
+      const props = resolveSceneObjectProps(targetAsset, target);
+      asset.props.width = Number(props.width);
+      asset.props.height = Number(props.height);
+    }
+    project.assets.push(asset);
+    scene.objects.push(createSceneObject(this.objectId, this.assetId, { x: 0, y: 0, z: 0 }, this.targetId));
+  }
+
+  undo(project: Project): void {
+    const scene = findScene(project, this.sceneId);
+    if (scene) scene.objects = scene.objects.filter((o) => o.id !== this.objectId);
+    project.assets = project.assets.filter((a) => a.id !== this.assetId);
+  }
+}