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/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;
         }