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.Core/Geometry/BoundingBox.cs

@@ -12,8 +12,8 @@ namespace OpenNest.Geometry
 
             double minX = boxes[0].X;
             double minY = boxes[0].Y;
-            double maxX = boxes[0].X + boxes[0].Width;
-            double maxY = boxes[0].Y + boxes[0].Length;
+            double maxX = boxes[0].Right;
+            double maxY = boxes[0].Top;
 
             foreach (var box in boxes)
             {