best times only improve; achievement unlock announces on the transition
only, so popup/Steam subscribers cannot double-fire; statistics are
increase-only (IncrementStat took any int).
RelockAchievement is console-only and documented as not reaching Steam -
a Steam achievement cannot be un-earned, so the disagreement is
permanent and deliberate.
refactor: extract EventLog, composed by both aggregates now that the
duplication actually exists. 34 tests.
