feat(progression): Profile aggregate (slice 4)

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.

progression/ProgressionEvent.cs

@@ -9,3 +9,9 @@ public abstract record ProgressionEvent;
 
 /// <summary>Raised when a Playthrough loses its last life.</summary>
 public sealed record RunEnded : ProgressionEvent;
+
+/// <summary>
+/// Raised the first time an Achievement is unlocked, and only then — so a subscriber that shows a
+/// popup or pushes to Steam cannot fire twice however often it is told.
+/// </summary>
+public sealed record AchievementUnlocked(AchievementId Achievement) : ProgressionEvent;