Review finding (Fix round 1, Task 1): the property getters read their
backing fields without synchronization while RecordSuccess/RecordFailure
write them under _gate, so a concurrent read could observe a torn
DateTimeOffset or a mix of fields from two different writes. Converted the
four auto-properties to explicit properties over private fields, each
getter taking the same lock the writers use. Public interface unchanged.
Joseph Doherty
adbf2eb182