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/Pattern.cs

@@ -19,26 +19,6 @@ namespace OpenNest
             BoundingBox = Parts.GetBoundingBox();
         }
 
-        public List<Line> GetLines(PushDirection facingDirection)
-        {
-            var lines = new List<Line>();
-
-            foreach (var part in Parts)
-                lines.AddRange(Helper.GetPartLines(part, facingDirection));
-
-            return lines;
-        }
-
-        public List<Line> GetOffsetLines(double spacing, PushDirection facingDirection)
-        {
-            var lines = new List<Line>();
-
-            foreach (var part in Parts)
-                lines.AddRange(Helper.GetOffsetPartLines(part, spacing, facingDirection));
-
-            return lines;
-        }
-
         public Pattern Clone(Vector offset)
         {
             var pattern = new Pattern();