feat(progression): snapshots, ports, in-memory adapters (slice 5)

flat snapshot record per aggregate - separate files mean saving a run
doesn't rewrite the Profile, deleting a slot is deleting a file, and a
corrupt slot doesn't cost achievements or best times.

snapshots hold primitives so Restore re-runs the value-object factories;
a serialiser building LevelId directly would bypass every guard.

LoadResult has three outcomes: Absent != Corrupt. Collapsing them is the
bug where a version bump made ContinueGame overwrite the save.

Profile.Restore writes collections directly - replaying UnlockAchievement
would re-fire popups and Steam pushes on every load. 44 tests.

progression/Profile.cs

@@ -57,6 +57,40 @@ public sealed class Profile
     /// </summary>
     public IReadOnlyList<ProgressionEvent> DrainEvents() => _events.Drain();
 
+    public ProfileSnapshot Export() => new(
+        _bestTimes.ToDictionary(e => e.Key.Value, e => e.Value.Seconds),
+        _achievements.Select(a => a.Value).ToList(),
+        _statistics.ToDictionary(e => e.Key.ToString(), e => e.Value));
+
+    /// <summary>
+    /// Rebuilds a Profile from a snapshot, re-running every value object's factory.
+    /// </summary>
+    /// <remarks>
+    /// Writes the collections directly rather than replaying <see cref="UnlockAchievement"/>. Going
+    /// through the operation would raise <see cref="AchievementUnlocked"/> for every achievement on
+    /// every load, so a subscriber pushing to Steam or showing a popup would fire on startup.
+    /// </remarks>
+    public static Profile Restore(ProfileSnapshot snapshot)
+    {
+        var profile = new Profile();
+
+        foreach (var (level, seconds) in snapshot.BestTimes)
+            profile._bestTimes[LevelId.Of(level)] = LevelTime.Of(seconds);
+
+        foreach (var achievement in snapshot.Achievements)
+            profile._achievements.Add(AchievementId.Of(achievement));
+
+        foreach (var (stat, value) in snapshot.Statistics)
+        {
+            // A statistic this build no longer tracks is dropped. Losing one counter beats refusing
+            // to load the Profile and with it every achievement and best time.
+            if (Enum.TryParse<StatName>(stat, out var known))
+                profile._statistics[known] = value;
+        }
+
+        return profile;
+    }
+
     public LevelTime? BestTimeFor(LevelId level) =>
         _bestTimes.TryGetValue(level, out var time) ? time : null;