Exposes public RecordFailure/RecordSuccess/GetRetryDelay (no-jitter, deterministic)
on MirrorCoordinator, plus RecordSourceSeq with HasGap/GapStart/GapEnd properties
and SetError/ClearError/HasError/ErrorMessage for error state. Makes IsDuplicate
and RecordMsgId public on SourceCoordinator and adds PruneDedupWindow(DateTimeOffset)
for explicit-cutoff dedup window pruning. Adds 5 unit tests in MirrorSourceRetryTests.
