…scene

Split the scene node query API into two explicit methods:

- root_nodes_in_scene: returns only the top-level nodes directly
  referenced by the scene (the original behavior)
- recursive_nodes_in_scene: returns all nodes including descendants
  in depth-first order

This fixes a bug where animations targeting child (parented) nodes
were silently skipped because only root nodes were collected.

Update call sites:
- Animator test now uses recursive_nodes_in_scene for correctness
- Example crate simplified by removing manual get_children helper

Co-authored-by: Andreas Streichardt <[email protected]>