feat(core): add CircleShape

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

OpenNest.Core/Shapes/CircleShape.cs

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+using System.Collections.Generic;
+using OpenNest.Geometry;
+
+namespace OpenNest.Shapes
+{
+    public class CircleShape : ShapeDefinition
+    {
+        public double Diameter { get; set; }
+
+        public override Drawing GetDrawing()
+        {
+            var entities = new List<Entity>
+            {
+                new Circle(0, 0, Diameter / 2.0)
+            };
+
+            return CreateDrawing(entities);
+        }
+    }
+}

OpenNest.Tests/Shapes/CircleShapeTests.cs

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+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);
+    }
+}