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>

tests/core/graph.test.ts

@@ -25,6 +25,7 @@ function context(overrides: Partial<NodeContext> = {}): NodeContext & { logs: st
     stopped: false,
     findByName: () => null,
     isKeyDown: () => false,
+    isTouching: () => false,
     log: (m: string) => logs.push(m),
     destroy: () => {},
     stop: () => { ctx.stopped = true; },