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/EventLog.cs

@@ -0,0 +1,19 @@
+namespace Mr.BrickAdventures.Progression;
+
+/// <summary>
+/// Collects the events an aggregate raises until someone drains them. Composed rather than
+/// inherited — aggregates own one, they are not a kind of it.
+/// </summary>
+internal sealed class EventLog
+{
+    private readonly List<ProgressionEvent> _events = new();
+
+    public void Raise(ProgressionEvent @event) => _events.Add(@event);
+
+    public IReadOnlyList<ProgressionEvent> Drain()
+    {
+        var drained = _events.ToList();
+        _events.Clear();
+        return drained;
+    }
+}