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/SaveEnvelope.cs

@@ -0,0 +1,10 @@
+namespace Mr.BrickAdventures.Progression.Adapters;
+
+/// <summary>
+/// What actually goes in a save file: a version alongside the snapshot.
+/// </summary>
+/// <remarks>
+/// The version lives here rather than in the snapshot record so the domain only ever knows the
+/// current shape. Migration is entirely this layer's problem.
+/// </remarks>
+internal sealed record SaveEnvelope<T>(int Version, T Data);