Function search_bm25 

pub fn search_bm25(
    query: &str,
    index: &Bm25Index,
    top_k: usize,
    selector: Option<&[usize]>,
) -> Vec<(usize, f32)>

Top-k BM25 search with optional selector mask and zero-score exclusion. Mirrors search.py:search_bm25.

Returns (chunk_index, score) pairs sorted by score descending.