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/MrGameEng.Sample.csproj

@@ -11,6 +11,7 @@
     <ProjectReference Include="..\..\src\MrGameEng.Input\MrGameEng.Input.csproj" />
     <ProjectReference Include="..\..\src\MrGameEng.Audio\MrGameEng.Audio.csproj" />
     <ProjectReference Include="..\..\src\MrGameEng.Assets\MrGameEng.Assets.csproj" />
+    <ProjectReference Include="..\..\src\MrGameEng.UI\MrGameEng.UI.csproj" />
     <ProjectReference Include="..\..\src\MrGameEng.Assets.Generator\MrGameEng.Assets.Generator.csproj"
                       OutputItemType="Analyzer" ReferenceOutputAssembly="false" />
   </ItemGroup>