feat: add EllipseConverter evaluation helpers with tests

Add EllipseConverter static class with foundational methods for converting
ellipse parameters to circular arcs: EvaluatePoint, EvaluateTangent,
EvaluateNormal, and IntersectNormals. All 8 unit tests pass.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

OpenNest.Tests/EllipseConverterTests.cs

@@ -0,0 +1,81 @@
+using OpenNest.Geometry;
+using OpenNest.Math;
+using Xunit;
+
+namespace OpenNest.Tests;
+
+public class EllipseConverterTests
+{
+    private const double Tol = 1e-10;
+
+    [Fact]
+    public void EvaluatePoint_AtZero_ReturnsMajorAxisEnd()
+    {
+        var p = EllipseConverter.EvaluatePoint(10, 5, 0, new Vector(0, 0), 0);
+        Assert.InRange(p.X, 10 - Tol, 10 + Tol);
+        Assert.InRange(p.Y, -Tol, Tol);
+    }
+
+    [Fact]
+    public void EvaluatePoint_AtHalfPi_ReturnsMinorAxisEnd()
+    {
+        var p = EllipseConverter.EvaluatePoint(10, 5, 0, new Vector(0, 0), System.Math.PI / 2);
+        Assert.InRange(p.X, -Tol, Tol);
+        Assert.InRange(p.Y, 5 - Tol, 5 + Tol);
+    }
+
+    [Fact]
+    public void EvaluatePoint_WithRotation_RotatesCorrectly()
+    {
+        var p = EllipseConverter.EvaluatePoint(10, 5, System.Math.PI / 2, new Vector(0, 0), 0);
+        Assert.InRange(p.X, -Tol, Tol);
+        Assert.InRange(p.Y, 10 - Tol, 10 + Tol);
+    }
+
+    [Fact]
+    public void EvaluatePoint_WithCenter_TranslatesCorrectly()
+    {
+        var p = EllipseConverter.EvaluatePoint(10, 5, 0, new Vector(100, 200), 0);
+        Assert.InRange(p.X, 110 - Tol, 110 + Tol);
+        Assert.InRange(p.Y, 200 - Tol, 200 + Tol);
+    }
+
+    [Fact]
+    public void EvaluateTangent_AtZero_PointsUp()
+    {
+        var t = EllipseConverter.EvaluateTangent(10, 5, 0, 0);
+        Assert.InRange(t.X, -Tol, Tol);
+        Assert.True(t.Y > 0);
+    }
+
+    [Fact]
+    public void EvaluateNormal_AtZero_PointsInward()
+    {
+        var n = EllipseConverter.EvaluateNormal(10, 5, 0, 0);
+        Assert.True(n.X < 0);
+        Assert.InRange(n.Y, -Tol, Tol);
+    }
+
+    [Fact]
+    public void IntersectNormals_PerpendicularNormals_FindsCenter()
+    {
+        var p1 = new Vector(5, 0);
+        var n1 = new Vector(-1, 0);
+        var p2 = new Vector(0, 5);
+        var n2 = new Vector(0, -1);
+        var center = EllipseConverter.IntersectNormals(p1, n1, p2, n2);
+        Assert.InRange(center.X, -Tol, Tol);
+        Assert.InRange(center.Y, -Tol, Tol);
+    }
+
+    [Fact]
+    public void IntersectNormals_ParallelNormals_ReturnsInvalid()
+    {
+        var p1 = new Vector(0, 0);
+        var n1 = new Vector(1, 0);
+        var p2 = new Vector(0, 5);
+        var n2 = new Vector(1, 0);
+        var center = EllipseConverter.IntersectNormals(p1, n1, p2, n2);
+        Assert.False(center.IsValid());
+    }
+}