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/ProfileSnapshot.cs

@@ -0,0 +1,14 @@
+namespace Mr.BrickAdventures.Progression;
+
+/// <summary>
+/// A Profile's state as plain data, for crossing the persistence seam. Primitives only, for the same
+/// reasons as <see cref="PlaythroughSnapshot"/>.
+/// </summary>
+/// <remarks>
+/// Statistics are keyed by <see cref="StatName"/>'s name rather than its numeric value, so
+/// reordering the enum cannot silently reassign a player's history.
+/// </remarks>
+public sealed record ProfileSnapshot(
+    IReadOnlyDictionary<string, double> BestTimes,
+    IReadOnlyList<string> Achievements,
+    IReadOnlyDictionary<string, int> Statistics);