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

src/MrGameEng.Graphics/SpriteSortKey.cs

@@ -0,0 +1,23 @@
+namespace MrGameEng.Graphics;
+
+/// <summary>
+/// Builds the 64-bit sort key the batcher orders sprites by:
+/// layer (8 bits) → depth (32 bits) → texture (24 bits).
+/// Texture bits only group equal textures for batching; collisions are harmless.
+/// </summary>
+public static class SpriteSortKey
+{
+    /// <summary>Composes a sort key from layer, depth and texture grouping key.</summary>
+    public static ulong Make(byte layer, float depth, int textureKey) =>
+        ((ulong)layer << 56) | ((ulong)DepthToSortableBits(depth) << 24) | ((uint)textureKey & 0xFF_FFFF);
+
+    /// <summary>
+    /// Maps a float to bits whose unsigned order matches the float order
+    /// (negative depths sort before positive ones).
+    /// </summary>
+    public static uint DepthToSortableBits(float depth)
+    {
+        var bits = BitConverter.SingleToUInt32Bits(depth);
+        return (bits & 0x8000_0000) != 0 ? ~bits : bits | 0x8000_0000;
+    }
+}