refactor(antigravity): document replay index lifecycle and align empty-contents semantics

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.