Function bm25_to_unit 

pub fn bm25_to_unit(raw: &[(String, f32)]) -> Vec<(String, f32)>

Normalize a batch of raw BM25-style scores to a bounded [0, 1] axis.

Scores are assumed to be higher-is-better by the time they reach this helper (FTS5 BM25 is negated at the search site). Empty batches return empty; all-zero batches return zero scores.