feat: add ConvexHull class with Andrew's monotone chain algorithm

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

OpenNest.Core/Geometry/ConvexHull.cs

@@ -0,0 +1,60 @@
+using System.Collections.Generic;
+using System.Linq;
+
+namespace OpenNest.Geometry
+{
+    public static class ConvexHull
+    {
+        public static Polygon Compute(IList<Vector> points)
+        {
+            if (points.Count < 3)
+            {
+                var result = new Polygon();
+                result.Vertices.AddRange(points);
+                return result;
+            }
+
+            var sorted = points.OrderBy(p => p.X).ThenBy(p => p.Y).ToList();
+
+            var lower = new List<Vector>();
+
+            foreach (var p in sorted)
+            {
+                while (lower.Count >= 2 && Cross(lower[lower.Count - 2], lower[lower.Count - 1], p) <= 0)
+                    lower.RemoveAt(lower.Count - 1);
+
+                lower.Add(p);
+            }
+
+            var upper = new List<Vector>();
+
+            for (int i = sorted.Count - 1; i >= 0; i--)
+            {
+                var p = sorted[i];
+
+                while (upper.Count >= 2 && Cross(upper[upper.Count - 2], upper[upper.Count - 1], p) <= 0)
+                    upper.RemoveAt(upper.Count - 1);
+
+                upper.Add(p);
+            }
+
+            // Remove last point of each half because it's repeated
+            lower.RemoveAt(lower.Count - 1);
+            upper.RemoveAt(upper.Count - 1);
+
+            lower.AddRange(upper);
+
+            var hull = new Polygon();
+            hull.Vertices.AddRange(lower);
+            hull.Close();
+            hull.UpdateBounds();
+
+            return hull;
+        }
+
+        private static double Cross(Vector o, Vector a, Vector b)
+        {
+            return (a.X - o.X) * (b.Y - o.Y) - (a.Y - o.Y) * (b.X - o.X);
+        }
+    }
+}