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/editor/App.ts

@@ -6,6 +6,7 @@ import { listScenes } from '../core/model/Project';
 import { HttpProjectStore } from '../core/store/HttpProjectStore';
 import type { ProjectStore } from '../core/store/ProjectStore';
 import { BehaviorSystem } from '../runtime/systems/BehaviorSystem';
+import { CollisionSystem } from '../runtime/systems/CollisionSystem';
 import { Input } from '../runtime/systems/Input';
 import { World } from '../runtime/World';
 import { Button, Dialog, HStack, Menu, type MenuItem, Splitter, Theme, ui, VStack, type Widget } from '../ui';
@@ -262,7 +263,10 @@ class Editor {
     ui().focus.blur();
     this.world = World.instantiate(this.state.project, scene.id);
     this.input.attach();
-    this.behaviors = new BehaviorSystem(this.state.project, this.input);
+    // Collisions are detected first so behaviors see this frame's enter/stay/exit events.
+    const collisions = new CollisionSystem();
+    this.behaviors = new BehaviorSystem(this.state.project, this.input, collisions);
+    this.world.addSystem(collisions);
     this.world.addSystem(this.behaviors);
     this.viewport.setWorld(this.world);
     this.state.setMode('play');