feat(core): add CircleShape

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

OpenNest.Tests/Shapes/CircleShapeTests.cs

@@ -0,0 +1,26 @@
+using OpenNest.Shapes;
+
+namespace OpenNest.Tests.Shapes;
+
+public class CircleShapeTests
+{
+    [Fact]
+    public void GetDrawing_ReturnsDrawingWithCorrectBoundingBox()
+    {
+        var shape = new CircleShape { Diameter = 10 };
+        var drawing = shape.GetDrawing();
+
+        var bbox = drawing.Program.BoundingBox();
+        Assert.Equal(10, bbox.Width, 0.01);
+        Assert.Equal(10, bbox.Length, 0.01);
+    }
+
+    [Fact]
+    public void GetDrawing_DefaultName_IsCircle()
+    {
+        var shape = new CircleShape { Diameter = 10 };
+        var drawing = shape.GetDrawing();
+
+        Assert.Equal("Circle", drawing.Name);
+    }
+}