Function build_selection_set 

pub fn build_selection_set(
    type_ref: &Value,
    types_by_name: &HashMap<String, Value>,
    depth: i32,
    seen: &mut HashSet<String>,
) -> String

Auto-generate a selection set (depth 2 = scalars + one nested object level). INTERFACE/UNION fields emit __typename (+ interface scalar fields when present).