feat(core): add RoundedRectangleShape

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

OpenNest.Tests/Shapes/RoundedRectangleShapeTests.cs

@@ -0,0 +1,38 @@
+using OpenNest.Shapes;
+
+namespace OpenNest.Tests.Shapes;
+
+public class RoundedRectangleShapeTests
+{
+    [Fact]
+    public void GetDrawing_BoundingBoxMatchesDimensions()
+    {
+        var shape = new RoundedRectangleShape { Width = 20, Height = 10, Radius = 2 };
+        var drawing = shape.GetDrawing();
+
+        var bbox = drawing.Program.BoundingBox();
+        Assert.Equal(20, bbox.Width, 0.1);
+        Assert.Equal(10, bbox.Length, 0.1);
+    }
+
+    [Fact]
+    public void GetDrawing_AreaIsLessThanFullRectangle()
+    {
+        var shape = new RoundedRectangleShape { Width = 20, Height = 10, Radius = 2 };
+        var drawing = shape.GetDrawing();
+
+        // Area should be less than 20*10=200 because corners are rounded
+        // Area = W*H - (4 - pi) * r^2 = 200 - (4 - pi) * 4 ~ 196.57
+        Assert.True(drawing.Area < 200);
+        Assert.True(drawing.Area > 190);
+    }
+
+    [Fact]
+    public void GetDrawing_ZeroRadius_MatchesRectangleArea()
+    {
+        var shape = new RoundedRectangleShape { Width = 20, Height = 10, Radius = 0 };
+        var drawing = shape.GetDrawing();
+
+        Assert.Equal(200, drawing.Area, 0.5);
+    }
+}