feat(core): add OctagonShape

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

OpenNest.Core/Shapes/OctagonShape.cs

@@ -0,0 +1,34 @@
+using System.Collections.Generic;
+using OpenNest.Geometry;
+
+namespace OpenNest.Shapes
+{
+    public class OctagonShape : ShapeDefinition
+    {
+        public double Width { get; set; }
+
+        public override Drawing GetDrawing()
+        {
+            var center = Width / 2.0;
+            var circumRadius = Width / (2.0 * System.Math.Cos(System.Math.PI / 8.0));
+
+            var vertices = new Vector[8];
+            for (var i = 0; i < 8; i++)
+            {
+                var angle = System.Math.PI / 8.0 + i * System.Math.PI / 4.0;
+                vertices[i] = new Vector(
+                    center + circumRadius * System.Math.Cos(angle),
+                    center + circumRadius * System.Math.Sin(angle));
+            }
+
+            var entities = new List<Entity>();
+            for (var i = 0; i < 8; i++)
+            {
+                var next = (i + 1) % 8;
+                entities.Add(new Line(vertices[i], vertices[next]));
+            }
+
+            return CreateDrawing(entities);
+        }
+    }
+}