fix: correct Width/Length axis mapping and add spiral center-fill

Box constructor and derived properties (Right, Top, Center, Translate, Offset)
had Width and Length swapped — Length is X axis, Width is Y axis. Corrected
across Core geometry, plate bounding box, rectangle packing, fill algorithms,
tests, and UI renderers.

Added FillSpiral with center remnant detection and recursive FillBest on
the gap between the 4 spiral quadrants. RectFill.FillBest now compares
spiral+center vs full best-fit fairly. BestCombination returns a
CombinationResult record instead of out params.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

OpenNest.Engine/DefaultNestEngine.cs

@@ -203,7 +203,7 @@ namespace OpenNest
             if (newWidth >= workArea.Width && newLength >= workArea.Length)
                 return workArea;
 
-            return new Box(workArea.X, workArea.Y, newWidth, newLength);
+            return new Box(workArea.X, workArea.Y, newLength, newWidth);
         }
 
         private List<Part> RunFillPipeline(NestItem item, Box workArea,