Function place_node 

pub fn place_node(
    tree: &mut NodeTree,
    id: &NodeId,
    new_parent: &NodeId,
    index: usize,
) -> Result<()>

Place id under new_parent at child index (clamped to the end). Unlike move_node this controls sibling order: index 0 makes it first. Refuses unknown nodes, root moves, and cycles (under itself or a descendant); the node keeps its subtree.