feat(core): add RectangleShape

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

OpenNest.Core/Shapes/RectangleShape.cs

@@ -0,0 +1,24 @@
+using System.Collections.Generic;
+using OpenNest.Geometry;
+
+namespace OpenNest.Shapes
+{
+    public class RectangleShape : ShapeDefinition
+    {
+        public double Width { get; set; }
+        public double Height { get; set; }
+
+        public override Drawing GetDrawing()
+        {
+            var entities = new List<Entity>
+            {
+                new Line(0, 0, Width, 0),
+                new Line(Width, 0, Width, Height),
+                new Line(Width, Height, 0, Height),
+                new Line(0, Height, 0, 0)
+            };
+
+            return CreateDrawing(entities);
+        }
+    }
+}