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.tests/InMemoryProfileStore.cs

@@ -0,0 +1,26 @@
+namespace Mr.BrickAdventures.Progression.Tests;
+
+internal sealed class InMemoryProfileStore : IProfileStore
+{
+    private ProfileSnapshot? _saved;
+    private bool _corrupt;
+
+    public void Save(Profile profile) => _saved = profile.Export();
+
+    public void Corrupt() => _corrupt = true;
+
+    public LoadResult<Profile> Load()
+    {
+        if (_corrupt) return LoadResult<Profile>.Corrupt("snapshot could not be read");
+        if (_saved is null) return LoadResult<Profile>.Absent();
+
+        try
+        {
+            return LoadResult<Profile>.Loaded(Profile.Restore(_saved));
+        }
+        catch (Exception e)
+        {
+            return LoadResult<Profile>.Corrupt(e.Message);
+        }
+    }
+}