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/ui/Theme.ts

@@ -32,6 +32,8 @@ export const Theme = {
     gizmo: 0x4c8dff,
     /** Behavior symbols and owner links. */
     behavior: 0xb066ff,
+    /** Collider outlines. */
+    collider: 0x4fd18b,
     /** Node graph: canvas, node chrome, per-category title colors, port colors, live highlight. */
     graphBackground: 0x15171b,
     node: 0x262a32,