Function query_top_nodes 

pub fn query_top_nodes(
    g: &ServeGraph,
    question: &str,
    top_k: usize,
    dense: Option<(&StaticEmbedder, &HashMap<String, Vec<f32>>)>,
) -> Vec<(String, String, String)>

Returns (node_id, label, source_file) for the top-K seeds from hybrid search.