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/AchievementId.cs

@@ -0,0 +1,21 @@
+namespace Mr.BrickAdventures.Progression;
+
+/// <summary>
+/// Stable identity of an Achievement, matching the authored <c>AchievementResource</c> id that Steam
+/// also keys on.
+/// </summary>
+public readonly struct AchievementId : IEquatable<AchievementId>
+{
+    public string Value { get; }
+
+    private AchievementId(string value) => Value = value;
+
+    public static AchievementId Of(string value) => string.IsNullOrWhiteSpace(value)
+        ? throw new ArgumentException("An achievement id cannot be blank.", nameof(value))
+        : new AchievementId(value);
+
+    public bool Equals(AchievementId other) => string.Equals(Value, other.Value, StringComparison.Ordinal);
+    public override bool Equals(object? obj) => obj is AchievementId other && Equals(other);
+    public override int GetHashCode() => StringComparer.Ordinal.GetHashCode(Value ?? string.Empty);
+    public override string ToString() => Value;
+}