Split the platform out of Core: new Host library + HeadlessHost

Core now depends only on Friflo.Engine.ECS — no MonoGame, no platform.
The windowed MonoGame host moves to the new MrGameEng.Host library:
GameHost, GameHostOptions, visual scene transitions (OverlayTransition,
Transitions.Fade/Wipe, TransitionRenderer) and the Input feature
(namespace MrGameEng.Input is unchanged). Transition timing stays in
Core (SceneManager exposes ActiveTransition/TransitionCoverage/
TransitionPhase; the host draws the overlay). EngineContext loses its
GraphicsDevice property: hosts publish the device as a service and
graphics code reads it via context.GetGraphicsDevice() in Graphics.

Core gains HeadlessHost: a fixed-timestep loop without a window or GPU
(Tick/RunTicks, Run with wall-clock pacing and lag resync) for dedicated
servers, batch simulation and tests. Graphics and Audio now carry their
own MonoGame.Framework.DesktopGL reference instead of inheriting it
from Core.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Directory.Build.props

@@ -1,5 +1,4 @@
 <Project>
-
   <PropertyGroup>
     <LangVersion>latest</LangVersion>
     <Nullable>enable</Nullable>
@@ -13,5 +12,4 @@
   <PropertyGroup Condition="$(MSBuildProjectName.StartsWith('MrGameEng.')) AND !$(MSBuildProjectName.EndsWith('.Tests')) AND !$(MSBuildProjectName.EndsWith('.Generator')) AND !$(MSBuildProjectName.EndsWith('.Sample'))">
     <GenerateDocumentationFile>true</GenerateDocumentationFile>
   </PropertyGroup>
-
 </Project>