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/Strategies/RectBestFitStrategy.cs

@@ -14,8 +14,7 @@ namespace OpenNest.Engine.Strategies
             var binItem = BinConverter.ToItem(context.Item, context.Plate.PartSpacing);
             var bin = BinConverter.CreateBin(context.WorkArea, context.Plate.PartSpacing);
 
-            var engine = new FillBestFit(bin);
-            engine.Fill(binItem);
+            RectFill.FillBest(bin, binItem);
 
             return BinConverter.ToParts(bin, new List<NestItem> { context.Item });
         }