Adds late-fusion hybrid search to Turnstone's log retrieval layer:
hybrid_score = 0.6 * bm25_normalized + 0.4 * cosine_similarity
Implementation:
- _bm25_search() extracts the existing FTS5 BM25 path as a named helper
- _hybrid_search() fetches an oversized BM25 candidate pool (5x limit,
min 100), embeds the query and each candidate text in-process via the
existing embeddings service, normalizes BM25 rank to [0,1], combines
with cosine similarity, and re-ranks
- search() gets semantic=False param that dispatches to _hybrid_search()
when True; pure BM25 remains the default for all existing call sites
- diagnose_stream() enables semantic=True so symptom-based queries
("database connection failed") surface semantically equivalent entries
("ECONNREFUSED", "backend gone away", "max retries exceeded")
- /api/search REST endpoint exposes ?semantic=true query param
Graceful degradation: falls back silently to pure BM25 when the embedding
backend is unavailable (EMBEDDING_AVAILABLE=False) or when embed_batch
raises an exception. No new infra — in-process numpy cosine, no vector DB.
11 new tests: BM25 helper, hybrid re-ranking, fallback paths, dispatcher.
372 + 11 = 383 tests passing.
Closes: #15
