Add MrGameEng.UI module integrating Myra

scene.UseUI() creates a per-scene Myra Desktop and registers
UiRenderSystem last in the draw phase (UI on top, window pixels).
GameHost now exposes itself as a Game service (Myra needs the instance;
useful for games too). Myra was picked over Gum/ImGui for the
FontStashSharp ecosystem fit, pipeline-free assets and maturity; its
internal SpriteBatch use is a documented exception to the engine rule.

Sample: on-screen HUD replaces window-title-only stats — live FPS label,
music volume slider and scene-switch buttons on both scenes.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/MrGameEng.Core/GameHost.cs

@@ -47,6 +47,7 @@ public class GameHost : Game
         Window.AllowUserResizing = _options.AllowResizing;
         Context.AttachGraphicsDevice(GraphicsDevice);
         Context.Services.Add(Window);
+        Context.Services.Add<Game>(this);
         base.Initialize();
         Context.Scenes.Switch(_initialScene);
     }