feat(progression): json stores on a real filesystem (slice 8)

new progression.adapters project, no engine reference. Stores take a
directory path; Godot resolves user:// with GlobalizePath and passes it
in, which is what makes them testable at all - originally I'd written
this slice off as untestable.

version lives in a file envelope, not the snapshot, so the domain only
knows the current shape. A newer-build save reports Corrupt with a
reason instead of masquerading as "no save" and getting overwritten.

atomic write via temp + rename. One file per aggregate: saving a run
leaves the profile file byte-identical, asserted.

guard rail fixed - it was matching prose in doc comments, now anchored
to using-directives plus a project.assets.json check. 63 tests.

progression.adapters/JsonProfileStore.cs

@@ -0,0 +1,20 @@
+namespace Mr.BrickAdventures.Progression.Adapters;
+
+/// <summary>
+/// Keeps the Profile in its own JSON file, beside the Playthrough files but written independently.
+/// </summary>
+public sealed class JsonProfileStore : IProfileStore
+{
+    /// <summary>Versioned separately from Playthroughs — the two shapes drift at their own pace.</summary>
+    private const int CurrentVersion = 1;
+
+    private readonly string _path;
+
+    public JsonProfileStore(string directory) => _path = Path.Combine(directory, "profile.json");
+
+    public void Save(Profile profile) =>
+        JsonFiles.WriteAtomic(_path, CurrentVersion, profile.Export());
+
+    public LoadResult<Profile> Load() =>
+        JsonFiles.Read<ProfileSnapshot, Profile>(_path, CurrentVersion, Profile.Restore);
+}