fix(geometry): add Entity.Clone() and stop NormalizeEntities from mutating originals

ShapeProfile.NormalizeEntities called Shape.Reverse() which flipped arc
directions on the original entity objects shared with the CAD view. Switching
to the Program tab and back would leave arcs reversed. Clone entities before
normalizing so the originals stay untouched.

Adds abstract Entity.Clone() with implementations on Line, Arc, Circle,
Polygon, and Shape (deep-clones children). Also adds CloneAll() extension
and replaces manual duplication in PartGeometry.CopyEntitiesAtLocation and
ProgramEditorControl.CloneEntity.

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

OpenNest.Core/Geometry/Entity.cs

@@ -251,6 +251,23 @@ namespace OpenNest.Geometry
         /// <returns></returns>
         public abstract bool Intersects(Shape shape, out List<Vector> pts);
 
+        /// <summary>
+        /// Creates a deep copy of the entity with a new Id.
+        /// </summary>
+        public abstract Entity Clone();
+
+        /// <summary>
+        /// Copies common Entity properties from this instance to the target.
+        /// </summary>
+        protected void CopyBaseTo(Entity target)
+        {
+            target.Color = Color;
+            target.Layer = Layer;
+            target.LineTypeName = LineTypeName;
+            target.IsVisible = IsVisible;
+            target.Tag = Tag;
+        }
+
         /// <summary>
         /// Type of entity.
         /// </summary>
@@ -259,6 +276,14 @@ namespace OpenNest.Geometry
 
     public static class EntityExtensions
     {
+        public static List<Entity> CloneAll(this IEnumerable<Entity> entities)
+        {
+            var result = new List<Entity>();
+            foreach (var e in entities)
+                result.Add(e.Clone());
+            return result;
+        }
+
         public static List<Vector> CollectPoints(this IEnumerable<Entity> entities)
         {
             var points = new List<Vector>();