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>

samples/MrGameEng.Sample/Scenes/StressScene.cs

@@ -4,6 +4,7 @@ using MrGameEng.Assets;
 using MrGameEng.Core;
 using MrGameEng.Graphics;
 using MrGameEng.Input;
+using MrGameEng.UI;
 
 namespace MrGameEng.Sample.Scenes;
 
@@ -52,10 +53,28 @@ public sealed class StressScene : Scene
 
         var camera = Store.CreateEntity(new Camera(Vector2.Zero, zoom: 0.3f));
 
+        var desktop = this.UseUI();
+        var statsLabel = new Myra.Graphics2D.UI.Label();
+        var backButton = new Myra.Graphics2D.UI.Button
+        {
+            Content = new Myra.Graphics2D.UI.Label { Text = "Back to main (Tab)" },
+        };
+        backButton.Click += (_, _) =>
+        {
+            if (!Context.Scenes.IsTransitioning)
+            {
+                Context.Scenes.Switch(new MainScene(), Transition.Wipe(0.8f, Color.DarkSlateBlue));
+            }
+        };
+        var panel = new Myra.Graphics2D.UI.VerticalStackPanel { Left = 12, Top = 12, Spacing = 6 };
+        panel.Widgets.Add(statsLabel);
+        panel.Widgets.Add(backButton);
+        desktop.Root = panel;
+
         UpdateSystems.Add(new BounceSystem(WorldBounds));
         UpdateSystems.Add(new StressCameraSystem(camera, input));
         UpdateSystems.Add(new SceneHotkeysSystem(Context, actions, () => new MainScene(), Transition.Wipe(0.8f, Color.DarkSlateBlue)));
-        UpdateSystems.Add(new TitleStatsSystem(Context, renderer, $"Stress {SpriteCount:N0}"));
+        UpdateSystems.Add(new StatsSystem(Context, renderer, $"Stress {SpriteCount:N0}", statsLabel));
     }
 
     /// <summary>Только зум колесом — чтобы регулировать число видимых спрайтов.</summary>