feat(core): add RingShape

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

OpenNest.Core/Shapes/RingShape.cs

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

OpenNest.Tests/Shapes/RingShapeTests.cs

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+using OpenNest.Shapes;
+
+namespace OpenNest.Tests.Shapes;
+
+public class RingShapeTests
+{
+    [Fact]
+    public void GetDrawing_BoundingBoxMatchesOuterDiameter()
+    {
+        var shape = new RingShape { OuterDiameter = 20, InnerDiameter = 10 };
+        var drawing = shape.GetDrawing();
+
+        var bbox = drawing.Program.BoundingBox();
+        Assert.Equal(20, bbox.Width, 0.01);
+        Assert.Equal(20, bbox.Length, 0.01);
+    }
+
+    [Fact]
+    public void GetDrawing_AreaExcludesInnerHole()
+    {
+        var shape = new RingShape { OuterDiameter = 20, InnerDiameter = 10 };
+        var drawing = shape.GetDrawing();
+
+        // Area = pi * (10^2 - 5^2) = pi * 75
+        var expectedArea = System.Math.PI * 75;
+        Assert.Equal(expectedArea, drawing.Area, 0.5);
+    }
+
+    [Fact]
+    public void GetDrawing_DefaultName_IsRing()
+    {
+        var shape = new RingShape { OuterDiameter = 20, InnerDiameter = 10 };
+        var drawing = shape.GetDrawing();
+
+        Assert.Equal("Ring", drawing.Name);
+    }
+}