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/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);
             }