Function fuse_retrieval 

pub fn fuse_retrieval(
    providers: &[&dyn RetrievalProvider],
    query: &RetrievalQuery,
) -> RetrievalResult<Vec<FusedRetrievalHit>>

Fuses provider ranks with deterministic integer reciprocal-rank fusion.

Scores from BM25 and vector stores are deliberately not compared directly. Providers, ties, duplicates, and final output all have stable ordering.

Errors

Propagates provider failures and rejects duplicate or empty provider names.