feat(core): add OctagonShape

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

OpenNest.Core/Shapes/OctagonShape.cs

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+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);
+        }
+    }
+}

OpenNest.Tests/Shapes/OctagonShapeTests.cs

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+using OpenNest.Shapes;
+
+namespace OpenNest.Tests.Shapes;
+
+public class OctagonShapeTests
+{
+    [Fact]
+    public void GetDrawing_BoundingBoxFitsWithinExpectedSize()
+    {
+        var shape = new OctagonShape { Width = 20 };
+        var drawing = shape.GetDrawing();
+
+        var bbox = drawing.Program.BoundingBox();
+        // Corner-to-corner is larger than flat-to-flat
+        Assert.True(bbox.Width >= 20 - 0.01);
+        Assert.True(bbox.Length >= 20 - 0.01);
+        // But should not be wildly larger (corner-to-corner ~ width / cos(22.5deg) ~ width * 1.0824)
+        Assert.True(bbox.Width < 22);
+        Assert.True(bbox.Length < 22);
+    }
+
+    [Fact]
+    public void GetDrawing_HasEightEdges()
+    {
+        var shape = new OctagonShape { Width = 20 };
+        var drawing = shape.GetDrawing();
+
+        // An octagon program should have 8 linear moves (one per edge)
+        var moves = drawing.Program.Codes
+            .OfType<OpenNest.CNC.LinearMove>()
+            .Count();
+        Assert.Equal(8, moves);
+    }
+}