feat(rendering): add Frustum with AABB intersection test

VoxelEngine.Tests/FrustumTests.cs

@@ -0,0 +1,44 @@
+using OpenTK.Mathematics;
+using VoxelEngine.Rendering;
+using Xunit;
+
+namespace VoxelEngine.Tests;
+
+public class FrustumTests
+{
+    // Camera at the origin, default yaw -90° → looks down -Z.
+    private static Frustum MakeFrustum()
+    {
+        var cam = new Camera(new Vector3(0, 0, 0), 16f / 9f);
+        return Frustum.FromViewProjection(cam.GetViewMatrix() * cam.GetProjectionMatrix());
+    }
+
+    [Fact]
+    public void AABBInFront_IsVisible()
+    {
+        var f = MakeFrustum();
+        Assert.True(f.Intersects(new Vector3(-2, -2, -22), new Vector3(2, 2, -18)));
+    }
+
+    [Fact]
+    public void AABBBehindCamera_IsCulled()
+    {
+        var f = MakeFrustum();
+        Assert.False(f.Intersects(new Vector3(-2, -2, 18), new Vector3(2, 2, 22)));
+    }
+
+    [Fact]
+    public void AABBStraddlingCamera_Intersects()
+    {
+        var f = MakeFrustum();
+        Assert.True(f.Intersects(new Vector3(-2, -2, -5), new Vector3(2, 2, 5)));
+    }
+
+    [Fact]
+    public void AABBFarToTheSide_IsCulled()
+    {
+        var f = MakeFrustum();
+        // Far along +X at a depth in front of the camera — outside horizontal FOV.
+        Assert.False(f.Intersects(new Vector3(1000, -2, -5), new Vector3(1004, 2, -1)));
+    }
+}