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/ProfileSnapshotTests.cs

@@ -0,0 +1,52 @@
+namespace Mr.BrickAdventures.Progression.Tests;
+
+public class ProfileSnapshotTests
+{
+    private static readonly LevelId Level = LevelId.Of("village_1");
+    private static readonly AchievementId RescueTen = AchievementId.Of("rescue_ten_kids");
+
+    [Fact]
+    public void A_profile_survives_a_round_trip_through_its_snapshot()
+    {
+        var profile = Profile.Empty();
+        profile.RecordTime(Level, LevelTime.Of(42.5));
+        profile.UnlockAchievement(RescueTen);
+        profile.Record(StatName.Deaths, 7);
+
+        var restored = Profile.Restore(profile.Export());
+
+        Assert.Equal(LevelTime.Of(42.5), restored.BestTimeFor(Level));
+        Assert.True(restored.HasAchievement(RescueTen));
+        Assert.Equal(7, restored.ReadStat(StatName.Deaths));
+    }
+
+    [Fact]
+    public void Restoring_a_profile_does_not_re_announce_its_achievements()
+    {
+        // If Restore replayed UnlockAchievement it would raise AchievementUnlocked for every
+        // achievement on every load — so the popup would fire on startup and Steam would be
+        // re-pushed each time. Loading is not earning.
+        var profile = Profile.Empty();
+        profile.UnlockAchievement(RescueTen);
+
+        var restored = Profile.Restore(profile.Export());
+
+        Assert.Empty(restored.DrainEvents());
+    }
+
+    [Fact]
+    public void An_unknown_statistic_is_dropped_rather_than_failing_the_load()
+    {
+        // A save written by a build that tracked a statistic this one no longer has. Losing one
+        // counter is acceptable; refusing to load the Profile — and with it every achievement and
+        // best time — is not.
+        var snapshot = new ProfileSnapshot(
+            BestTimes: new Dictionary<string, double>(),
+            Achievements: Array.Empty<string>(),
+            Statistics: new Dictionary<string, int> { ["Deaths"] = 4, ["BricksThrown"] = 99 });
+
+        var restored = Profile.Restore(snapshot);
+
+        Assert.Equal(4, restored.ReadStat(StatName.Deaths));
+    }
+}