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/Playthrough.cs

@@ -122,4 +122,35 @@ public sealed class Playthrough
     /// once after each use case and republishes.
     /// </summary>
     public IReadOnlyList<ProgressionEvent> DrainEvents() => _events.Drain();
+
+    public PlaythroughSnapshot Export() => new(
+        Id.Value,
+        Lives.Value,
+        Bank.Value,
+        CurrentLevel.Value,
+        _ownedSkills.ToDictionary(e => e.Key.Value, e => e.Value.Value),
+        _clearedLevels.Select(l => l.Value).ToList());
+
+    /// <summary>
+    /// Rebuilds a Playthrough from a snapshot, re-running every value object's factory so an invalid
+    /// snapshot is rejected here rather than becoming corrupt in-memory state.
+    /// </summary>
+    public static Playthrough Restore(PlaythroughSnapshot snapshot)
+    {
+        var playthrough = new Playthrough(
+            PlaythroughId.Of(snapshot.Id),
+            LevelId.Of(snapshot.CurrentLevel),
+            Lives.Of(snapshot.Lives))
+        {
+            Bank = Coins.Of(snapshot.Bank),
+        };
+
+        foreach (var (skill, level) in snapshot.OwnedSkills)
+            playthrough._ownedSkills[SkillId.Of(skill)] = SkillLevel.Of(level);
+
+        foreach (var level in snapshot.ClearedLevels)
+            playthrough._clearedLevels.Add(LevelId.Of(level));
+
+        return playthrough;
+    }
 }