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/graph/NodeRegistry.ts

@@ -51,6 +51,11 @@ export interface NodeContext {
   dt: number;
   findByName(name: string): EntityHandle | null;
   isKeyDown(key: string): boolean;
+  /**
+   * Whether any collider of `a` overlaps any collider of `b` right now. A collider entity
+   * stands for itself; any other entity stands for its direct collider children.
+   */
+  isTouching(a: EntityHandle, b: EntityHandle): boolean;
   log(message: string): void;
   destroy(target: EntityHandle): void;
   stop(): void;