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.UI/MrGameEng.UI.csproj

@@ -0,0 +1,15 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>net8.0</TargetFramework>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="Myra" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\MrGameEng.Core\MrGameEng.Core.csproj" />
+  </ItemGroup>
+
+</Project>

src/MrGameEng.UI/SceneUiExtensions.cs

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+using Friflo.Engine.ECS.Systems;
+using Microsoft.Xna.Framework;
+using Myra;
+using Myra.Graphics2D.UI;
+using MrGameEng.Core;
+
+namespace MrGameEng.UI;
+
+/// <summary>
+/// Draw system rendering the scene's Myra <see cref="Desktop"/>. Must run after the scene's
+/// world rendering (register UI last in the draw phase) — the UI is drawn on top in window
+/// pixels and also processes mouse/keyboard interaction during render.
+/// </summary>
+public sealed class UiRenderSystem : BaseSystem
+{
+    private readonly Desktop _desktop;
+
+    /// <summary>Creates the system for <paramref name="desktop"/>.</summary>
+    public UiRenderSystem(Desktop desktop) => _desktop = desktop;
+
+    /// <inheritdoc />
+    protected override void OnUpdateGroup() => _desktop.Render();
+}
+
+/// <summary>Wires the UI module (Myra) into a <see cref="Scene"/>.</summary>
+public static class SceneUiExtensions
+{
+    private static bool _environmentInitialized;
+
+    /// <summary>
+    /// Creates a Myra <see cref="Desktop"/> for this scene and registers
+    /// <see cref="UiRenderSystem"/> in the draw phase. Call from <c>OnLoad</c>
+    /// <b>after</b> <c>UseRenderer2D()</c> so the UI draws on top of the world.
+    /// Build the UI by assigning <see cref="Desktop.Root"/>.
+    /// </summary>
+    public static Desktop UseUI(this Scene scene)
+    {
+        // Геттер MyraEnvironment.Game бросает исключение, пока Game не задан — проверять через ??= нельзя.
+        if (!_environmentInitialized)
+        {
+            MyraEnvironment.Game = scene.Context.Services.Get<Game>();
+            _environmentInitialized = true;
+        }
+
+        var desktop = new Desktop();
+        scene.DrawSystems.Add(new UiRenderSystem(desktop));
+        return desktop;
+    }
+}