Spawn node: place assets at runtime into the play copy, Spawner demo behavior

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

src/runtime/systems/BehaviorSystem.ts

@@ -161,6 +161,10 @@ export class BehaviorSystem implements System {
         world.log.push(message);
         console.log(`[behavior ${entity.name}]`, message);
       },
+      spawn: (assetId, position, parent) => {
+        const parentEntity = parent ? world.entities.find((e) => e === parent || e.id === parent.id) ?? null : null;
+        return world.spawn(assetId, position, parentEntity);
+      },
       destroy: (target: EntityHandle) => {
         const found = world.entities.find((e) => e === target || e.id === target.id);
         if (found) world.remove(found);