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/PlaythroughSnapshot.cs

@@ -0,0 +1,19 @@
+namespace Mr.BrickAdventures.Progression;
+
+/// <summary>
+/// A Playthrough's state as plain data, for crossing the persistence seam.
+/// </summary>
+/// <remarks>
+/// Deliberately primitives rather than value objects. A serialiser constructing <see cref="LevelId"/>
+/// or <see cref="Coins"/> directly would bypass their factories and every guard in them, so instead
+/// <see cref="Playthrough.Restore"/> re-runs those factories and a corrupt file fails at the domain
+/// edge. Carries no version field — versioning and migration belong to the store adapter, so the
+/// domain only ever knows the current shape.
+/// </remarks>
+public sealed record PlaythroughSnapshot(
+    Guid Id,
+    int Lives,
+    int Bank,
+    string CurrentLevel,
+    IReadOnlyDictionary<string, int> OwnedSkills,
+    IReadOnlyList<string> ClearedLevels);