feat(core): add ShapeDefinition base class

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

OpenNest.Core/Shapes/ShapeDefinition.cs

@@ -0,0 +1,33 @@
+using System;
+using System.Collections.Generic;
+using OpenNest.Converters;
+using OpenNest.Geometry;
+
+namespace OpenNest.Shapes
+{
+    public abstract class ShapeDefinition
+    {
+        public string Name { get; set; }
+
+        protected ShapeDefinition()
+        {
+            var typeName = GetType().Name;
+            Name = typeName.EndsWith("Shape")
+                ? typeName.Substring(0, typeName.Length - 5)
+                : typeName;
+        }
+
+        public abstract Drawing GetDrawing();
+
+        protected Drawing CreateDrawing(List<Entity> entities)
+        {
+            var pgm = ConvertGeometry.ToProgram(entities);
+
+            if (pgm == null)
+                throw new InvalidOperationException(
+                    $"Failed to create program for shape '{Name}'. Check that parameters produce valid geometry.");
+
+            return new Drawing(Name, pgm);
+        }
+    }
+}