feat: add ArcCandidate and Kasa circle fitting

Foundation for the geometry simplifier that will replace consecutive line
segments with fitted arcs. Adds ArcCandidate data class, GeometrySimplifier
with stub Analyze/Apply methods, and FitCircle using the Kasa algebraic
least-squares method. Also adds InternalsVisibleTo for OpenNest.Tests on
OpenNest.Core.

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

OpenNest.Tests/GeometrySimplifierTests.cs

@@ -0,0 +1,43 @@
+using OpenNest.Geometry;
+using Xunit;
+
+namespace OpenNest.Tests;
+
+public class GeometrySimplifierTests
+{
+    [Fact]
+    public void FitCircle_PointsOnKnownCircle_ReturnsCorrectCenterAndRadius()
+    {
+        // 21 points on a semicircle centered at (5, 3) with radius 10
+        var center = new Vector(5, 3);
+        var radius = 10.0;
+        var points = new List<Vector>();
+        for (var i = 0; i <= 20; i++)
+        {
+            var angle = i * System.Math.PI / 20;
+            points.Add(new Vector(
+                center.X + radius * System.Math.Cos(angle),
+                center.Y + radius * System.Math.Sin(angle)));
+        }
+
+        var (fitCenter, fitRadius) = GeometrySimplifier.FitCircle(points);
+
+        Assert.InRange(fitCenter.X, 4.999, 5.001);
+        Assert.InRange(fitCenter.Y, 2.999, 3.001);
+        Assert.InRange(fitRadius, 9.999, 10.001);
+    }
+
+    [Fact]
+    public void FitCircle_CollinearPoints_ReturnsInvalidCenter()
+    {
+        // Collinear points should produce degenerate result
+        var points = new List<Vector>
+        {
+            new(0, 0), new(1, 0), new(2, 0), new(3, 0), new(4, 0)
+        };
+
+        var (fitCenter, _) = GeometrySimplifier.FitCircle(points);
+
+        Assert.False(fitCenter.IsValid());
+    }
+}