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/Input/InputSystem.cs

@@ -0,0 +1,46 @@
+using Friflo.Engine.ECS.Systems;
+using MrGameEng.Core;
+
+namespace MrGameEng.Input;
+
+/// <summary>Polls the <see cref="InputManager"/> once per frame. Registered first in the update phase.</summary>
+public sealed class InputSystem : BaseSystem
+{
+    private readonly InputManager _input;
+
+    /// <summary>Creates the system for <paramref name="input"/>.</summary>
+    public InputSystem(InputManager input) => _input = input;
+
+    /// <inheritdoc />
+    protected override void OnUpdateGroup() => _input.Update();
+}
+
+/// <summary>Wires the input module into a <see cref="Scene"/>.</summary>
+public static class SceneInputExtensions
+{
+    /// <summary>
+    /// Returns the shared <see cref="InputManager"/> service (creating it on first use) and
+    /// inserts <see cref="InputSystem"/> at the start of the scene's update phase.
+    /// Call from <c>OnLoad</c> before adding gameplay systems.
+    /// </summary>
+    public static InputManager UseInput(this Scene scene)
+    {
+        var services = scene.Context.Services;
+        var input = services.GetOrDefault<InputManager>();
+        if (input is null)
+        {
+            var capture = services.GetOrDefault<InputCapture>();
+            if (capture is null)
+            {
+                capture = new InputCapture();
+                services.Add(capture);
+            }
+
+            input = new InputManager(capture);
+            services.Add(input);
+        }
+
+        scene.UpdateSystems.Insert(0, new InputSystem(input));
+        return input;
+    }
+}