refactor: clean up NestEngine — collapse overloads, extract helper, remove dead code

- Fill(NestItem) and Fill(List<Part>) now delegate to their Box overloads
- Add Part.CreateAtOrigin() to replace repeated 4-line build-at-origin pattern
  used in NestEngine, RotationSlideStrategy, and PairEvaluator
- Remove dead code: FillArea overloads, Fill(NestItem, int), FillWithPairs(NestItem),
  ConvertTileResultToParts, PackBottomLeft.FindPointHorizontal, Pattern.GetLines/GetOffsetLines,
  unused count variable in FillNoRotation
- Simplify IsBetterValidFill to delegate to IsBetterFill after overlap check

NestEngine reduced from 717 to 484 lines.

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

OpenNest.Core/Part.cs

@@ -103,6 +103,23 @@ namespace OpenNest
             BoundingBox.Offset(voffset);
         }
 
+        /// <summary>
+        /// Creates a part normalized to the origin with optional rotation.
+        /// </summary>
+        public static Part CreateAtOrigin(Drawing drawing, double rotation = 0)
+        {
+            var part = new Part(drawing);
+
+            if (!Math.Tolerance.IsEqualTo(rotation, 0))
+                part.Rotate(rotation);
+
+            var bbox = part.Program.BoundingBox();
+            part.Offset(-bbox.Location.X, -bbox.Location.Y);
+            part.UpdateBounds();
+
+            return part;
+        }
+
         /// <summary>
         /// Updates the bounding box of the part.
         /// </summary>