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

src/MrGameEng.Graphics/Layers.cs

@@ -0,0 +1,62 @@
+namespace MrGameEng.Graphics;
+
+/// <summary>Compact identifier of a render layer. Obtained from <see cref="LayerRegistry.Register"/>.</summary>
+public readonly record struct LayerId(byte Value)
+{
+    /// <summary>The default layer (the first one registered).</summary>
+    public static readonly LayerId Default = new(0);
+}
+
+/// <summary>Coordinate space a layer is drawn in.</summary>
+public enum LayerSpace
+{
+    /// <summary>Drawn through the active camera's transform.</summary>
+    World,
+
+    /// <summary>Drawn in screen coordinates, ignoring the camera (HUD, UI). Never culled.</summary>
+    Screen,
+}
+
+/// <summary>How sprites are ordered within a layer.</summary>
+public enum LayerSortMode
+{
+    /// <summary>Order by the sprite's <see cref="Sprite.Depth"/> value (smaller = drawn first).</summary>
+    Depth,
+
+    /// <summary>Order by world Y position (top-down games: lower on screen = drawn in front).</summary>
+    YSort,
+}
+
+/// <summary>A registered render layer.</summary>
+public sealed record RenderLayer(LayerId Id, string Name, LayerSpace Space, LayerSortMode SortMode);
+
+/// <summary>
+/// Registry of render layers. Layers are registered up front (typically when the renderer is
+/// created) and drawn in registration order. Maximum 256 layers.
+/// </summary>
+public sealed class LayerRegistry
+{
+    private readonly List<RenderLayer> _layers = [];
+
+    /// <summary>Creates a registry containing the built-in "Default" world layer.</summary>
+    public LayerRegistry() => Register("Default");
+
+    /// <summary>Number of registered layers.</summary>
+    public int Count => _layers.Count;
+
+    /// <summary>Registers a layer drawn after all previously registered ones.</summary>
+    public LayerId Register(string name, LayerSpace space = LayerSpace.World, LayerSortMode sortMode = LayerSortMode.Depth)
+    {
+        if (_layers.Count == 256)
+        {
+            throw new InvalidOperationException("Maximum number of render layers (256) reached.");
+        }
+
+        var id = new LayerId((byte)_layers.Count);
+        _layers.Add(new RenderLayer(id, name, space, sortMode));
+        return id;
+    }
+
+    /// <summary>Returns the layer with the given id.</summary>
+    public RenderLayer this[LayerId id] => _layers[id.Value];
+}