Function flatten_layout_nodes 

fn flatten_layout_nodes(
    node: &LayoutNode,
    depth: usize,
    index_path: &str,
    out: &mut Vec<Value>,
)

Flatten one layout tree into a compact, renderable node list.

Deliberately a FLAT list carrying a depth, not nested JSON: the CLI renders an indented outline, and a flat list keeps both the wire shape and the renderer trivial.