perf: add CloneAtOffset to skip re-rotation and bbox walk on part clones

Part.Clone() re-clones from the drawing's unrotated program, re-rotates,
and walks all CNC codes twice for bounding box — 4 O(c) passes per clone.
CloneAtOffset clones from the already-rotated program and computes the
bounding box arithmetically, reducing to 1 O(c) pass per clone.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

OpenNest.Engine/Pattern.cs

@@ -24,11 +24,7 @@ namespace OpenNest
             var pattern = new Pattern();
 
             foreach (var part in Parts)
-            {
-                var clone = (Part)part.Clone();
-                clone.Offset(offset);
-                pattern.Parts.Add(clone);
-            }
+                pattern.Parts.Add(part.CloneAtOffset(offset));
 
             pattern.UpdateBounds();
             return pattern;