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/InMemoryPlaythroughStore.cs

@@ -0,0 +1,34 @@
+namespace Mr.BrickAdventures.Progression.Tests;
+
+/// <summary>
+/// The test-side adapter for <see cref="IPlaythroughStore"/>. Its existence alongside the JSON store
+/// is what makes the port a real seam rather than a hypothetical one.
+/// </summary>
+internal sealed class InMemoryPlaythroughStore : IPlaythroughStore
+{
+    private readonly Dictionary<Guid, PlaythroughSnapshot> _saved = new();
+    private readonly HashSet<Guid> _corrupt = new();
+
+    public void Save(Playthrough playthrough) => _saved[playthrough.Id.Value] = playthrough.Export();
+
+    /// <summary>Simulates a file that exists but cannot be read back into a valid snapshot.</summary>
+    public void Corrupt(PlaythroughId id) => _corrupt.Add(id.Value);
+
+    public LoadResult<Playthrough> Load(PlaythroughId id)
+    {
+        if (_corrupt.Contains(id.Value))
+            return LoadResult<Playthrough>.Corrupt("snapshot could not be read");
+
+        if (!_saved.TryGetValue(id.Value, out var snapshot))
+            return LoadResult<Playthrough>.Absent();
+
+        try
+        {
+            return LoadResult<Playthrough>.Loaded(Playthrough.Restore(snapshot));
+        }
+        catch (Exception e)
+        {
+            return LoadResult<Playthrough>.Corrupt(e.Message);
+        }
+    }
+}