Search Relevance: From BM25 to Hybrid

Vector search did not replace keyword search; production search runs both, because they fail in opposite places. This path builds relevance engineering from the ground up: the inverted index and the analysis decisions baked into it, BM25 and its saturation arithmetic, hybrid retrieval with reciprocal rank fusion, and the evaluation machinery, judgment sets, NDCG, position bias, that tells you whether any of it actually got better.