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/SampleSystems.cs

@@ -113,8 +113,9 @@ public sealed class SceneHotkeysSystem(
     }
 }
 
-/// <summary>FPS и статистика рендера в заголовке окна (обновляется 4 раза в секунду).</summary>
-public sealed class TitleStatsSystem(EngineContext context, Renderer2D renderer, string sceneName) : BaseSystem
+/// <summary>FPS и статистика рендера: в HUD-лейбл и в заголовок окна (4 раза в секунду).</summary>
+public sealed class StatsSystem(
+    EngineContext context, Renderer2D renderer, string sceneName, Myra.Graphics2D.UI.Label? hudLabel = null) : BaseSystem
 {
     private float _accumulated;
     private int _frames;
@@ -131,8 +132,13 @@ public sealed class TitleStatsSystem(EngineContext context, Renderer2D renderer,
         var fps = _frames / _accumulated;
         _accumulated = 0f;
         _frames = 0;
-        context.Services.Get<GameWindow>().Title =
-            $"MrGameEng Sample — {sceneName} | {fps:F0} FPS | sprites: {renderer.SubmittedSprites} | culled: {renderer.CulledSprites} | draw calls: {renderer.DrawCalls}";
+        var stats =
+            $"{fps:F0} FPS | sprites: {renderer.SubmittedSprites} | culled: {renderer.CulledSprites} | draw calls: {renderer.DrawCalls}";
+        context.Services.Get<GameWindow>().Title = $"MrGameEng Sample — {sceneName} | {stats}";
+        if (hudLabel is not null)
+        {
+            hudLabel.Text = $"{sceneName}\n{stats}";
+        }
     }
 }