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/LoadResult.cs

@@ -0,0 +1,26 @@
+namespace Mr.BrickAdventures.Progression;
+
+public enum LoadOutcome
+{
+    Loaded,
+    Absent,
+    Corrupt,
+}
+
+/// <summary>
+/// The result of loading something from a store: it was there, it was never there, or it was there
+/// and unreadable.
+/// </summary>
+/// <remarks>
+/// Three outcomes rather than two, deliberately. The system being replaced collapsed "no save file"
+/// and "version mismatch" into a single <c>false</c> from <c>LoadGame</c>, which <c>ContinueGame</c>
+/// then read as "no save" — so it started a new game and overwrote the player's file. Absent means
+/// offer a new game; Corrupt means tell the player something went wrong.
+/// </remarks>
+public sealed record LoadResult<T>(LoadOutcome Outcome, T? Value, string? Problem)
+    where T : class
+{
+    public static LoadResult<T> Loaded(T value) => new(LoadOutcome.Loaded, value, null);
+    public static LoadResult<T> Absent() => new(LoadOutcome.Absent, null, null);
+    public static LoadResult<T> Corrupt(string problem) => new(LoadOutcome.Corrupt, null, problem);
+}