Document that the replay request index is request-scoped, retains no-copy GJSON results aliasing the payload, and is not safe for concurrent use, and that the context fingerprints snapshot a running hash lazily in content order. Replace the fingerprint byte counter with a boolean since it only distinguishes an empty fingerprint from a hashed one. Guard reasoningReplayItemsFromRequest on validContents so a valid but empty contents array keeps returning an empty non-nil slice, exactly as it did before the index was introduced, and lock that invariant with a test.