Distributed checkpoint

Simulated message bus, deterministic: implement class IdempotentProcessor with int Process(string messageId, Func<int> work) — processes work once per messageId, caches the result, returns it on repeats WITHOUT re-running work (work has a side-effect counter the test watches). Then implement static int[] DeliverAll(List<string> messageIds, Func<string, int> handler, int duplicates) that delivers every message duplicates times (out of order, shuffled deterministically by index) and returns the work-counter delta — which must be exactly messageIds.Count because duplicates are absorbed idempotently.