Design and implement an in-memory key-value store supporting set plus transactional begin, commit, and abort.

Store values in a base map and keep a stack of transaction overlays or undo logs. Reads search newest to oldest; commit merges the top layer, rollback discards it, and TTL uses explicit timestamps plus lazy or scheduled cleanup.

Test empty input, one-element input, duplicates, boundary indices, invalid states, and the largest allowed size; state time and space complexity.