feat(geometry): add PolyLabel algorithm with square test

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

OpenNest.Tests/PolyLabelTests.cs

@@ -0,0 +1,27 @@
+using OpenNest.Geometry;
+
+namespace OpenNest.Tests;
+
+public class PolyLabelTests
+{
+    private static Polygon Square(double size)
+    {
+        var p = new Polygon();
+        p.Vertices.Add(new Vector(0, 0));
+        p.Vertices.Add(new Vector(size, 0));
+        p.Vertices.Add(new Vector(size, size));
+        p.Vertices.Add(new Vector(0, size));
+        return p;
+    }
+
+    [Fact]
+    public void Square_ReturnsCenterPoint()
+    {
+        var poly = Square(100);
+
+        var result = PolyLabel.Find(poly);
+
+        Assert.Equal(50, result.X, 1.0);
+        Assert.Equal(50, result.Y, 1.0);
+    }
+}