Update README.md to include project description, developer documentation links, and license information.

tests/MrGameEng.Core.Tests/SceneManagerTests.cs

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+using MrGameEng.Core;
+using Xunit;
+
+namespace MrGameEng.Core.Tests;
+
+public class SceneManagerTests
+{
+    private sealed class TrackingScene : Scene
+    {
+        public int LoadCount;
+        public int UnloadCount;
+        public int UpdateCount;
+
+        protected override void OnLoad() => LoadCount++;
+
+        protected override void OnUnload() => UnloadCount++;
+
+        public override void Update(GameClock clock)
+        {
+            UpdateCount++;
+            base.Update(clock);
+        }
+    }
+
+    [Fact]
+    public void Switch_IsDeferred_UntilNextUpdate()
+    {
+        var context = new EngineContext();
+        var scene = new TrackingScene();
+
+        context.Scenes.Switch(scene);
+
+        Assert.Null(context.Scenes.Current);
+        Assert.Equal(0, scene.LoadCount);
+
+        context.Scenes.Update(context.Clock);
+
+        Assert.Same(scene, context.Scenes.Current);
+        Assert.Equal(1, scene.LoadCount);
+        Assert.Equal(1, scene.UpdateCount);
+        Assert.True(scene.IsLoaded);
+    }
+
+    [Fact]
+    public void Switch_UnloadsPreviousScene_AndLoadsNext()
+    {
+        var context = new EngineContext();
+        var first = new TrackingScene();
+        var second = new TrackingScene();
+
+        context.Scenes.Switch(first);
+        context.Scenes.Update(context.Clock);
+        context.Scenes.Switch(second);
+        context.Scenes.Update(context.Clock);
+
+        Assert.Equal(1, first.UnloadCount);
+        Assert.False(first.IsLoaded);
+        Assert.Same(second, context.Scenes.Current);
+        Assert.Equal(1, second.LoadCount);
+    }
+
+    [Fact]
+    public void Switch_ToNull_UnloadsCurrentScene()
+    {
+        var context = new EngineContext();
+        var scene = new TrackingScene();
+
+        context.Scenes.Switch(scene);
+        context.Scenes.Update(context.Clock);
+        context.Scenes.Switch(null);
+        context.Scenes.Update(context.Clock);
+
+        Assert.Null(context.Scenes.Current);
+        Assert.Equal(1, scene.UnloadCount);
+    }
+
+    [Fact]
+    public void Update_WithoutScene_DoesNothing()
+    {
+        var context = new EngineContext();
+
+        context.Scenes.Update(context.Clock);
+        context.Scenes.Draw(context.Clock);
+
+        Assert.Null(context.Scenes.Current);
+    }
+}