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/InputManagerTests.cs

@@ -0,0 +1,106 @@
+using Microsoft.Xna.Framework;
+using Microsoft.Xna.Framework.Input;
+using MrGameEng.Input;
+using Xunit;
+
+namespace MrGameEng.Input.Tests;
+
+public class InputManagerTests
+{
+    private static MouseState Mouse(
+        int x = 0,
+        int y = 0,
+        int wheel = 0,
+        ButtonState left = ButtonState.Released
+    ) =>
+        new(
+            x,
+            y,
+            wheel,
+            left,
+            ButtonState.Released,
+            ButtonState.Released,
+            ButtonState.Released,
+            ButtonState.Released
+        );
+
+    private static void Frame(
+        InputManager input,
+        KeyboardState keyboard = default,
+        MouseState mouse = default
+    ) => input.Apply(keyboard, mouse, GamePadState.Default);
+
+    [Fact]
+    public void KeyPressed_OnlyOnTheFrameItGoesDown()
+    {
+        var input = new InputManager();
+
+        Frame(input, new KeyboardState(Keys.Space));
+        Assert.True(input.IsKeyPressed(Keys.Space));
+        Assert.True(input.IsKeyDown(Keys.Space));
+
+        Frame(input, new KeyboardState(Keys.Space));
+        Assert.False(input.IsKeyPressed(Keys.Space));
+        Assert.True(input.IsKeyDown(Keys.Space));
+    }
+
+    [Fact]
+    public void KeyReleased_OnlyOnTheFrameItGoesUp()
+    {
+        var input = new InputManager();
+
+        Frame(input, new KeyboardState(Keys.A));
+        Frame(input);
+
+        Assert.True(input.IsKeyReleased(Keys.A));
+        Frame(input);
+        Assert.False(input.IsKeyReleased(Keys.A));
+    }
+
+    [Fact]
+    public void Capture_SuppressesInput_KeepsMousePositionAndWheel()
+    {
+        var capture = new MrGameEng.Core.InputCapture();
+        var input = new InputManager(capture);
+
+        Frame(input, new KeyboardState(Keys.W), Mouse(x: 10, y: 20, wheel: 120));
+        Assert.True(input.IsKeyDown(Keys.W));
+
+        capture.Captured = true;
+        input.Update(); // ввод захвачен оверлеем — устройства не опрашиваются
+
+        Assert.False(input.IsKeyDown(Keys.W));
+        Assert.True(input.IsKeyReleased(Keys.W)); // одно корректное событие отпускания
+        Assert.Equal(new Point(10, 20), input.MousePosition);
+        Assert.Equal(0, input.WheelDelta);
+        Assert.Equal(Point.Zero, input.MouseDelta);
+
+        input.Update();
+        Assert.False(input.IsKeyReleased(Keys.W)); // и больше никаких фантомных событий
+    }
+
+    [Fact]
+    public void MouseDeltaAndWheelDelta_ComputedBetweenFrames()
+    {
+        var input = new InputManager();
+
+        Frame(input, mouse: Mouse(x: 10, y: 10, wheel: 0));
+        Frame(input, mouse: Mouse(x: 25, y: 5, wheel: 120));
+
+        Assert.Equal(new Point(15, -5), input.MouseDelta);
+        Assert.Equal(120, input.WheelDelta);
+        Assert.Equal(new Point(25, 5), input.MousePosition);
+    }
+
+    [Fact]
+    public void MousePressed_DetectsLeftButtonEdge()
+    {
+        var input = new InputManager();
+
+        Frame(input, mouse: Mouse());
+        Frame(input, mouse: Mouse(left: ButtonState.Pressed));
+
+        Assert.True(input.IsMousePressed(MouseButton.Left));
+        Assert.False(input.IsMousePressed(MouseButton.Right));
+    }
+}