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/Playthrough.cs

@@ -11,7 +11,7 @@ namespace Mr.BrickAdventures.Progression;
 /// </remarks>
 public sealed class Playthrough
 {
-    private readonly List<ProgressionEvent> _events = new();
+    private readonly EventLog _events = new();
 
     private Playthrough(PlaythroughId id, LevelId currentLevel, Lives lives)
     {
@@ -114,17 +114,12 @@ public sealed class Playthrough
         Lives = Lives.Decrement();
 
         if (Lives.IsExhausted)
-            _events.Add(new RunEnded());
+            _events.Raise(new RunEnded());
     }
 
     /// <summary>
     /// Takes the events raised since the last drain, clearing them. The composition root drains
     /// once after each use case and republishes.
     /// </summary>
-    public IReadOnlyList<ProgressionEvent> DrainEvents()
-    {
-        var drained = _events.ToList();
-        _events.Clear();
-        return drained;
-    }
+    public IReadOnlyList<ProgressionEvent> DrainEvents() => _events.Drain();
 }