Cache layer for rerank results. Key is SHA256 of query + sorted
candidate IDs, so changing the candidate set automatically invalidates.
1-day TTL by default; corrupted cache files are silently dropped.
5 tests cover hit, miss, different-candidates, TTL expiry, corruption.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
