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

tests/MrGameEng.Graphics.Tests/CullingTests.cs

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+using Microsoft.Xna.Framework;
+using MrGameEng.Graphics;
+using Xunit;
+
+namespace MrGameEng.Graphics.Tests;
+
+public class CullingTests
+{
+    [Fact]
+    public void BoundingCircle_UnrotatedTopLeftOrigin_CenterIsRegionCenter()
+    {
+        var transform = Transform2D.At(new Vector2(100f, 200f));
+
+        var (center, radius) = CullingMath.SpriteBoundingCircle(transform, 32f, 32f, Vector2.Zero);
+
+        Assert.Equal(new Vector2(116f, 216f), center);
+        Assert.Equal(0.5f * MathF.Sqrt(32f * 32f * 2f), radius, 3);
+    }
+
+    [Fact]
+    public void BoundingCircle_CenterOrigin_CenterIsPosition()
+    {
+        var transform = Transform2D.At(new Vector2(50f, 50f));
+
+        var (center, _) = CullingMath.SpriteBoundingCircle(transform, 64f, 32f, new Vector2(32f, 16f));
+
+        Assert.Equal(new Vector2(50f, 50f), center);
+    }
+
+    [Fact]
+    public void BoundingCircle_Scale_GrowsRadius()
+    {
+        var transform = new Transform2D(Vector2.Zero, scale: new Vector2(2f, 2f));
+
+        var (_, radius) = CullingMath.SpriteBoundingCircle(transform, 10f, 10f, Vector2.Zero);
+
+        Assert.Equal(0.5f * MathF.Sqrt(800f), radius, 3);
+    }
+
+    [Theory]
+    [InlineData(50f, 50f, true)]    // inside
+    [InlineData(-4f, 50f, true)]    // touching from the left (radius 5)
+    [InlineData(-20f, 50f, false)]  // far left
+    [InlineData(50f, 130f, false)]  // far below
+    public void CircleIntersectsRect_DetectsOverlap(float x, float y, bool expected)
+    {
+        var rect = new RectF(0f, 0f, 100f, 100f);
+
+        Assert.Equal(expected, CullingMath.CircleIntersectsRect(new Vector2(x, y), 5f, rect));
+    }
+}