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.Host/TransitionRenderer.cs

@@ -0,0 +1,63 @@
+using Microsoft.Xna.Framework;
+using Microsoft.Xna.Framework.Graphics;
+
+namespace MrGameEng.Host;
+
+/// <summary>
+/// Minimal overlay renderer handed to <see cref="OverlayTransition.Draw"/>: fills rectangles
+/// in normalized screen coordinates (0..1 on both axes) over the rendered scene.
+/// </summary>
+public sealed class TransitionRenderer : IDisposable
+{
+    private readonly GraphicsDevice _device;
+    private readonly BasicEffect _effect;
+    private readonly VertexPositionColor[] _vertices = new VertexPositionColor[6];
+
+    /// <summary>Disposes the GPU effect. Called by <see cref="GameHost"/> on shutdown.</summary>
+    public void Dispose() => _effect.Dispose();
+
+    internal TransitionRenderer(GraphicsDevice device)
+    {
+        _device = device;
+        _effect = new BasicEffect(device)
+        {
+            VertexColorEnabled = true,
+            TextureEnabled = false,
+            World = Matrix.Identity,
+            View = Matrix.Identity,
+            Projection = Matrix.CreateOrthographicOffCenter(0f, 1f, 1f, 0f, 0f, 1f),
+        };
+    }
+
+    /// <summary>
+    /// Fills a rectangle given in normalized screen coordinates with <paramref name="color"/>
+    /// at the given <paramref name="opacity"/> (0 = invisible, 1 = solid).
+    /// </summary>
+    public void Fill(float x, float y, float width, float height, Color color, float opacity = 1f)
+    {
+        var alpha = (byte)(Math.Clamp(opacity, 0f, 1f) * color.A);
+        var premultiplied = Color.FromNonPremultiplied(color.R, color.G, color.B, alpha);
+
+        var topLeft = new Vector3(x, y, 0f);
+        var topRight = new Vector3(x + width, y, 0f);
+        var bottomLeft = new Vector3(x, y + height, 0f);
+        var bottomRight = new Vector3(x + width, y + height, 0f);
+
+        _vertices[0] = new VertexPositionColor(topLeft, premultiplied);
+        _vertices[1] = new VertexPositionColor(topRight, premultiplied);
+        _vertices[2] = new VertexPositionColor(bottomLeft, premultiplied);
+        _vertices[3] = new VertexPositionColor(bottomLeft, premultiplied);
+        _vertices[4] = new VertexPositionColor(topRight, premultiplied);
+        _vertices[5] = new VertexPositionColor(bottomRight, premultiplied);
+
+        _device.BlendState = BlendState.AlphaBlend;
+        _device.DepthStencilState = DepthStencilState.None;
+        _device.RasterizerState = RasterizerState.CullNone;
+
+        foreach (var pass in _effect.CurrentTechnique.Passes)
+        {
+            pass.Apply();
+            _device.DrawUserPrimitives(PrimitiveType.TriangleList, _vertices, 0, 2);
+        }
+    }
+}