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.Core/Part.cs

@@ -208,5 +208,29 @@ namespace OpenNest
 
             return part;
         }
+
+        /// <summary>
+        /// Creates an offset copy of the part. Clones from the already-rotated
+        /// program (skips re-rotation) and computes the bounding box arithmetically
+        /// (skips Program.BoundingBox walk).
+        /// </summary>
+        public Part CloneAtOffset(Vector offset)
+        {
+            var clonedProgram = Program.Clone() as Program;
+            var part = new Part(BaseDrawing, clonedProgram,
+                location + offset,
+                new Box(BoundingBox.X + offset.X, BoundingBox.Y + offset.Y,
+                    BoundingBox.Width, BoundingBox.Height));
+
+            return part;
+        }
+
+        private Part(Drawing baseDrawing, Program program, Vector location, Box boundingBox)
+        {
+            BaseDrawing = baseDrawing;
+            Program = program;
+            this.location = location;
+            BoundingBox = boundingBox;
+        }
     }
 }

OpenNest.Engine/FillLinear.cs

@@ -76,8 +76,7 @@ namespace OpenNest
             var pushDir = GetPushDirection(direction);
             var opposite = Helper.OppositeDirection(pushDir);
 
-            var partB = (Part)partA.Clone();
-            partB.Offset(MakeOffset(direction, bboxDim));
+            var partB = partA.CloneAtOffset(MakeOffset(direction, bboxDim));
 
             var movingLines = boundary.GetLines(partB.Location, pushDir);
             var stationaryLines = boundary.GetLines(partA.Location, opposite);
@@ -364,8 +363,7 @@ namespace OpenNest
                 if (nextPos + dim > limit + Tolerance.Epsilon)
                     break;
 
-                var clone = (Part)template.Clone();
-                clone.Offset(MakeOffset(direction, copyDistance * count));
+                var clone = template.CloneAtOffset(MakeOffset(direction, copyDistance * count));
                 seed.Parts.Add(clone);
                 count++;
             }

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;