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

src/MrGameEng.Core/SceneManager.cs

@@ -0,0 +1,51 @@
+namespace MrGameEng.Core;
+
+/// <summary>
+/// Owns the active <see cref="Scene"/>. Scene switches are deferred to the start of the
+/// next update so a scene is never unloaded in the middle of its own frame.
+/// </summary>
+public sealed class SceneManager
+{
+    /// <summary>The active scene, or null before the first switch is applied.</summary>
+    public Scene? Current { get; private set; }
+
+    private readonly EngineContext _context;
+    private Scene? _pending;
+    private bool _hasPending;
+
+    internal SceneManager(EngineContext context) => _context = context;
+
+    /// <summary>
+    /// Requests a switch to <paramref name="scene"/>. The current scene is unloaded and the new
+    /// one loaded at the start of the next update tick. Passing null unloads the current scene.
+    /// </summary>
+    public void Switch(Scene? scene)
+    {
+        _pending = scene;
+        _hasPending = true;
+    }
+
+    /// <summary>Applies a pending switch, then updates the active scene. Called by the host.</summary>
+    public void Update(GameClock clock)
+    {
+        ApplyPending();
+        Current?.Update(clock);
+    }
+
+    /// <summary>Draws the active scene. Called by the host.</summary>
+    public void Draw(GameClock clock) => Current?.Draw(clock);
+
+    internal void ApplyPending()
+    {
+        if (!_hasPending)
+        {
+            return;
+        }
+
+        _hasPending = false;
+        Current?.Unload();
+        Current = _pending;
+        _pending = null;
+        Current?.Load(_context);
+    }
+}