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/HorizontalRemnantEngine.cs

@@ -38,7 +38,7 @@ namespace OpenNest
             var bb = item.Drawing.Program.BoundingBox();
             var cos = System.Math.Abs(System.Math.Cos(angle));
             var sin = System.Math.Abs(System.Math.Sin(angle));
-            return bb.Length * cos + bb.Width * sin;
+            return bb.Width * cos + bb.Length * sin;
         }
     }
 }