feat(core): add RoundedRectangleShape

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

OpenNest.Core/Shapes/RoundedRectangleShape.cs

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+using System.Collections.Generic;
+using OpenNest.Geometry;
+using OpenNest.Math;
+
+namespace OpenNest.Shapes
+{
+    public class RoundedRectangleShape : ShapeDefinition
+    {
+        public double Width { get; set; }
+        public double Height { get; set; }
+        public double Radius { get; set; }
+
+        public override Drawing GetDrawing()
+        {
+            var r = Radius;
+            var entities = new List<Entity>();
+
+            if (r <= 0)
+            {
+                entities.Add(new Line(0, 0, Width, 0));
+                entities.Add(new Line(Width, 0, Width, Height));
+                entities.Add(new Line(Width, Height, 0, Height));
+                entities.Add(new Line(0, Height, 0, 0));
+            }
+            else
+            {
+                // Bottom edge (left to right, above bottom-left arc to bottom-right arc)
+                entities.Add(new Line(r, 0, Width - r, 0));
+
+                // Bottom-right corner arc: center at (Width-r, r), from 270deg to 360deg
+                entities.Add(new Arc(Width - r, r, r,
+                    Angle.ToRadians(270), Angle.ToRadians(360)));
+
+                // Right edge
+                entities.Add(new Line(Width, r, Width, Height - r));
+
+                // Top-right corner arc: center at (Width-r, Height-r), from 0deg to 90deg
+                entities.Add(new Arc(Width - r, Height - r, r,
+                    Angle.ToRadians(0), Angle.ToRadians(90)));
+
+                // Top edge (right to left)
+                entities.Add(new Line(Width - r, Height, r, Height));
+
+                // Top-left corner arc: center at (r, Height-r), from 90deg to 180deg
+                entities.Add(new Arc(r, Height - r, r,
+                    Angle.ToRadians(90), Angle.ToRadians(180)));
+
+                // Left edge
+                entities.Add(new Line(0, Height - r, 0, r));
+
+                // Bottom-left corner arc: center at (r, r), from 180deg to 270deg
+                entities.Add(new Arc(r, r, r,
+                    Angle.ToRadians(180), Angle.ToRadians(270)));
+            }
+
+            return CreateDrawing(entities);
+        }
+    }
+}

OpenNest.Tests/Shapes/RoundedRectangleShapeTests.cs

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+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);
+    }
+}