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/Arc.cs

@@ -267,6 +267,13 @@ namespace OpenNest.Geometry
             get { return Diameter * System.Math.PI * SweepAngle() / Angle.TwoPI; }
         }
 
+        public override Entity Clone()
+        {
+            var copy = new Arc(center, radius, startAngle, endAngle, reversed);
+            CopyBaseTo(copy);
+            return copy;
+        }
+
         /// <summary>
         /// Reverses the rotation direction.
         /// </summary>

OpenNest.Core/Geometry/Circle.cs

@@ -165,6 +165,13 @@ namespace OpenNest.Geometry
             get { return Circumference(); }
         }
 
+        public override Entity Clone()
+        {
+            var copy = new Circle(center, radius) { Rotation = Rotation };
+            CopyBaseTo(copy);
+            return copy;
+        }
+
         /// <summary>
         /// Reverses the rotation direction.
         /// </summary>

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>();

OpenNest.Core/Geometry/Line.cs

@@ -257,6 +257,13 @@ namespace OpenNest.Geometry
             }
         }
 
+        public override Entity Clone()
+        {
+            var copy = new Line(pt1, pt2);
+            CopyBaseTo(copy);
+            return copy;
+        }
+
         /// <summary>
         /// Reversed the line.
         /// </summary>

OpenNest.Core/Geometry/Polygon.cs

@@ -168,6 +168,13 @@ namespace OpenNest.Geometry
             get { return Perimeter(); }
         }
 
+        public override Entity Clone()
+        {
+            var copy = new Polygon { Vertices = new List<Vector>(Vertices) };
+            CopyBaseTo(copy);
+            return copy;
+        }
+
         /// <summary>
         /// Reverses the rotation direction of the polygon.
         /// </summary>

OpenNest.Core/Geometry/Shape.cs

@@ -349,6 +349,15 @@ namespace OpenNest.Geometry
             return polygon;
         }
 
+        public override Entity Clone()
+        {
+            var copy = new Shape();
+            foreach (var e in Entities)
+                copy.Entities.Add(e.Clone());
+            CopyBaseTo(copy);
+            return copy;
+        }
+
         /// <summary>
         /// Reverses the rotation direction of the shape.
         /// </summary>

OpenNest.Core/Geometry/ShapeProfile.cs

@@ -75,7 +75,8 @@ namespace OpenNest.Geometry
         /// </summary>
         public static List<Entity> NormalizeEntities(IEnumerable<Entity> entities)
         {
-            var profile = new ShapeProfile(entities.ToList());
+            var cloned = entities.CloneAll();
+            var profile = new ShapeProfile(cloned);
             return profile.ToNormalizedEntities();
         }
 

OpenNest.Core/PartGeometry.cs

@@ -126,20 +126,10 @@ namespace OpenNest
         {
             var result = new List<Entity>(source.Count);
 
-            for (var i = 0; i < source.Count; i++)
+            foreach (var entity in source)
             {
-                var entity = source[i];
-                Entity copy;
-
-                if (entity is Line line)
-                    copy = new Line(line.StartPoint + location, line.EndPoint + location);
-                else if (entity is Arc arc)
-                    copy = new Arc(arc.Center + location, arc.Radius, arc.StartAngle, arc.EndAngle, arc.IsReversed);
-                else if (entity is Circle circle)
-                    copy = new Circle(circle.Center + location, circle.Radius);
-                else
-                    continue;
-
+                var copy = entity.Clone();
+                copy.Offset(location);
                 result.Add(copy);
             }
 

OpenNest/Controls/ProgramEditorControl.cs

@@ -209,15 +209,8 @@ namespace OpenNest.Controls
 
         private static Entity CloneEntity(Entity entity, Color color)
         {
-            Entity clone = entity switch
-            {
-                Line line => new Line(line.StartPoint, line.EndPoint) { Layer = line.Layer, IsVisible = line.IsVisible },
-                Arc arc => new Arc(arc.Center, arc.Radius, arc.StartAngle, arc.EndAngle, arc.IsReversed) { Layer = arc.Layer, IsVisible = arc.IsVisible },
-                Circle circle => new Circle(circle.Center, circle.Radius) { Layer = circle.Layer, IsVisible = circle.IsVisible },
-                _ => null,
-            };
-            if (clone != null)
-                clone.Color = color;
+            var clone = entity.Clone();
+            clone.Color = color;
             return clone;
         }