Function replace_node 

pub fn replace_node(
    root: &mut LayoutNode,
    target_id: &str,
    new_node: LayoutNode,
) -> Result<(), LayoutError>

Replace the node at target_id with new_node, preserving the target’s size. If target is root, replaces root-in-place (caller’s Option<LayoutNode> doesn’t change reference — the fields are updated on the root node).