feat(core): add OctagonShape

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

OpenNest.Tests/Shapes/OctagonShapeTests.cs

@@ -0,0 +1,34 @@
+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);
+    }
+}