Function build_scene_node 

fn build_scene_node(
    entity: EntityId,
    info: &HashMap<EntityId, NodeInfo>,
    children_of: &HashMap<EntityId, Vec<EntityId>>,
    visited: &mut HashSet<EntityId>,
) -> Option<SceneNode>

Builds the SceneNode for entity and, recursively, its children.

visited guards against a malformed Parent chain looping back on itself: a cycle would otherwise recurse until the stack blew. Returns None for an entity already placed in the tree, which is what breaks the loop.