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/PlaythroughStoreTests.cs

@@ -0,0 +1,47 @@
+namespace Mr.BrickAdventures.Progression.Tests;
+
+public class PlaythroughStoreTests
+{
+    private static readonly PlaythroughId Id =
+        PlaythroughId.Of(new Guid("33333333-3333-3333-3333-333333333333"));
+
+    private static Campaign OneLevel() => Campaign.Of(new[] { LevelId.Of("village_1") });
+
+    [Fact]
+    public void A_playthrough_that_was_never_saved_is_absent_not_an_error()
+    {
+        var store = new InMemoryPlaythroughStore();
+
+        var result = store.Load(Id);
+
+        Assert.Equal(LoadOutcome.Absent, result.Outcome);
+    }
+
+    [Fact]
+    public void A_playthrough_that_cannot_be_read_is_corrupt_and_not_mistaken_for_absent()
+    {
+        // This is the version-bump bug. LoadGame returns false for both "no file" and "version
+        // mismatch", ContinueGame reads that as no save, calls StartNewGame, and overwrites the
+        // player's file. Absent means "offer a new game"; Corrupt means "say something went wrong".
+        var store = new InMemoryPlaythroughStore();
+        store.Save(Playthrough.StartNew(Id, OneLevel(), Lives.Of(3)));
+        store.Corrupt(Id);
+
+        var result = store.Load(Id);
+
+        Assert.Equal(LoadOutcome.Corrupt, result.Outcome);
+        Assert.NotEqual(LoadOutcome.Absent, result.Outcome);
+    }
+
+    [Fact]
+    public void A_saved_playthrough_loads_back()
+    {
+        var store = new InMemoryPlaythroughStore();
+        store.Save(Playthrough.StartNew(Id, OneLevel(), Lives.Of(2)));
+
+        var result = store.Load(Id);
+
+        Assert.Equal(LoadOutcome.Loaded, result.Outcome);
+        Assert.Equal(2, result.Value!.Lives.Value);
+    }
+}