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>

src/MrGameEng.Core/SceneManager.cs

@@ -22,13 +22,24 @@ public sealed class SceneManager
     /// <summary>True while a transition is covering or revealing.</summary>
     public bool IsTransitioning => _state != State.Idle;
 
+    /// <summary>The transition currently covering or revealing, or null while idle.
+    /// Hosts that render overlays read this together with <see cref="TransitionCoverage"/>
+    /// and <see cref="TransitionPhase"/> during their draw phase.</summary>
+    public Transition? ActiveTransition => _state == State.Idle ? null : _transition;
+
+    /// <summary>Coverage of the active transition: 0 = scene fully visible, 1 = fully covered.</summary>
+    public float TransitionCoverage => Math.Clamp(_coverage, 0f, 1f);
+
+    /// <summary>Phase of the active transition. Meaningful only while <see cref="IsTransitioning"/>.</summary>
+    public TransitionPhase TransitionPhase =>
+        _state == State.CoveringOut ? TransitionPhase.Out : TransitionPhase.In;
+
     private readonly EngineContext _context;
     private Scene? _pending;
     private bool _hasPending;
     private Transition? _transition;
     private State _state;
     private float _coverage;
-    private TransitionRenderer? _renderer;
 
     internal SceneManager(EngineContext context) => _context = context;
 
@@ -100,27 +111,9 @@ public sealed class SceneManager
         Current?.Update(clock);
     }
 
-    /// <summary>Draws the active scene and the transition overlay on top. Called by the host.</summary>
-    public void Draw(GameClock clock)
-    {
-        Current?.Draw(clock);
-
-        if (_state == State.Idle || !_context.HasGraphicsDevice)
-        {
-            return;
-        }
-
-        _renderer ??= new TransitionRenderer(_context.GraphicsDevice);
-        var phase = _state == State.CoveringOut ? TransitionPhase.Out : TransitionPhase.In;
-        _transition!.Draw(_renderer, Math.Clamp(_coverage, 0f, 1f), phase);
-    }
-
-    /// <summary>Disposes the lazily created transition renderer. Called on host shutdown.</summary>
-    internal void DisposeRenderer()
-    {
-        _renderer?.Dispose();
-        _renderer = null;
-    }
+    /// <summary>Draws the active scene. Transition overlays are rendered by the host on top,
+    /// from <see cref="ActiveTransition"/> and <see cref="TransitionCoverage"/>.</summary>
+    public void Draw(GameClock clock) => Current?.Draw(clock);
 
     internal void ApplyPending()
     {