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

src/MrGameEng.Core/Scene.cs

@@ -0,0 +1,62 @@
+using Friflo.Engine.ECS;
+using Friflo.Engine.ECS.Systems;
+
+namespace MrGameEng.Core;
+
+/// <summary>
+/// A scene owns its ECS world (<see cref="EntityStore"/>) and two system roots:
+/// <see cref="UpdateSystems"/> for game logic and <see cref="DrawSystems"/> for rendering.
+/// Override <see cref="OnLoad"/> to create entities and register systems.
+/// </summary>
+public abstract class Scene
+{
+    /// <summary>The ECS world of this scene.</summary>
+    public EntityStore Store { get; } = new();
+
+    /// <summary>Systems executed every update tick, in registration order.</summary>
+    public SystemRoot UpdateSystems { get; }
+
+    /// <summary>Systems executed every draw tick, in registration order.</summary>
+    public SystemRoot DrawSystems { get; }
+
+    /// <summary>Engine context. Valid from <see cref="OnLoad"/> until <see cref="OnUnload"/>.</summary>
+    public EngineContext Context => _context ?? throw new InvalidOperationException("Scene is not loaded.");
+
+    /// <summary>True while the scene is the active, loaded scene.</summary>
+    public bool IsLoaded => _context is not null;
+
+    private EngineContext? _context;
+
+    /// <summary>Initializes the scene's ECS world and system roots.</summary>
+    protected Scene()
+    {
+        UpdateSystems = new SystemRoot(Store, "Update");
+        DrawSystems = new SystemRoot(Store, "Draw");
+    }
+
+    /// <summary>Called once when the scene becomes active: create entities, add systems.</summary>
+    protected abstract void OnLoad();
+
+    /// <summary>Called once when the scene is replaced or the game exits. Release scene resources here.</summary>
+    protected virtual void OnUnload() { }
+
+    /// <summary>Runs the update phase. Called by <see cref="SceneManager"/>.</summary>
+    public virtual void Update(GameClock clock) =>
+        UpdateSystems.Update(new UpdateTick(clock.DeltaTime, (float)clock.TotalTime));
+
+    /// <summary>Runs the draw phase. Called by <see cref="SceneManager"/>.</summary>
+    public virtual void Draw(GameClock clock) =>
+        DrawSystems.Update(new UpdateTick(clock.DeltaTime, (float)clock.TotalTime));
+
+    internal void Load(EngineContext context)
+    {
+        _context = context;
+        OnLoad();
+    }
+
+    internal void Unload()
+    {
+        OnUnload();
+        _context = null;
+    }
+}