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>

Directory.Packages.props

@@ -6,6 +6,7 @@
     <PackageVersion Include="Friflo.Engine.ECS" Version="3.6.0" />
     <PackageVersion Include="FontStashSharp.MonoGame" Version="1.5.6" />
     <PackageVersion Include="NVorbis" Version="0.10.5" />
+    <PackageVersion Include="Myra" Version="1.6.1" />
 
     <!-- Source generator -->
     <PackageVersion Include="Microsoft.CodeAnalysis.CSharp" Version="4.8.0" />