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.Engine/RectanglePacking/FillNoRotation.cs

@@ -17,7 +17,6 @@ namespace OpenNest.RectanglePacking
         {
             var ycount = (int)System.Math.Floor((Bin.Height + Tolerance.Epsilon) / item.Height);
             var xcount = (int)System.Math.Floor((Bin.Width + Tolerance.Epsilon) / item.Width);
-            var count = ycount * xcount;
 
             for (int i = 0; i < xcount; i++)
             {