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>

tests/MrGameEng.Host.Tests/Input/ActionMapTests.cs

@@ -0,0 +1,75 @@
+using Microsoft.Xna.Framework.Input;
+using MrGameEng.Input;
+using Xunit;
+
+namespace MrGameEng.Input.Tests;
+
+public class ActionMapTests
+{
+    private enum GameAction
+    {
+        Jump,
+        MoveLeft,
+        MoveRight,
+    }
+
+    private static void Frame(InputManager input, params Keys[] keys) =>
+        input.Apply(new KeyboardState(keys), default, GamePadState.Default);
+
+    [Fact]
+    public void IsDown_TrueWhenAnyBindingIsHeld()
+    {
+        var input = new InputManager();
+        var map = new ActionMap<GameAction>(input)
+            .Bind(GameAction.Jump, Keys.Space)
+            .Bind(GameAction.Jump, Keys.W);
+
+        Frame(input, Keys.W);
+
+        Assert.True(map.IsDown(GameAction.Jump));
+    }
+
+    [Fact]
+    public void IsPressed_EdgeTriggered()
+    {
+        var input = new InputManager();
+        var map = new ActionMap<GameAction>(input).Bind(GameAction.Jump, Keys.Space);
+
+        Frame(input, Keys.Space);
+        Assert.True(map.IsPressed(GameAction.Jump));
+
+        Frame(input, Keys.Space);
+        Assert.False(map.IsPressed(GameAction.Jump));
+        Assert.True(map.IsDown(GameAction.Jump));
+    }
+
+    [Fact]
+    public void Unbind_RemovesAllBindings()
+    {
+        var input = new InputManager();
+        var map = new ActionMap<GameAction>(input).Bind(GameAction.Jump, Keys.Space);
+
+        map.Unbind(GameAction.Jump);
+        Frame(input, Keys.Space);
+
+        Assert.False(map.IsDown(GameAction.Jump));
+    }
+
+    [Fact]
+    public void GetAxis_CombinesTwoActions()
+    {
+        var input = new InputManager();
+        var map = new ActionMap<GameAction>(input)
+            .Bind(GameAction.MoveLeft, Keys.A)
+            .Bind(GameAction.MoveRight, Keys.D);
+
+        Frame(input, Keys.A);
+        Assert.Equal(-1f, map.GetAxis(GameAction.MoveLeft, GameAction.MoveRight));
+
+        Frame(input, Keys.A, Keys.D);
+        Assert.Equal(0f, map.GetAxis(GameAction.MoveLeft, GameAction.MoveRight));
+
+        Frame(input, Keys.D);
+        Assert.Equal(1f, map.GetAxis(GameAction.MoveLeft, GameAction.MoveRight));
+    }
+}