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/IPlaythroughStore.cs

@@ -0,0 +1,12 @@
+namespace Mr.BrickAdventures.Progression;
+
+/// <summary>
+/// Where Playthroughs are kept. One per slot; the JSON adapter writes one file each, so saving a run
+/// never rewrites the Profile and deleting a slot is deleting a file.
+/// </summary>
+public interface IPlaythroughStore
+{
+    void Save(Playthrough playthrough);
+
+    LoadResult<Playthrough> Load(PlaythroughId id);
+}