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.Tests/Shapes/RectangleShapeTests.cs

@@ -11,8 +11,8 @@ public class RectangleShapeTests
         var drawing = shape.GetDrawing();
 
         var bbox = drawing.Program.BoundingBox();
-        Assert.Equal(10, bbox.Width, 0.01);
-        Assert.Equal(5, bbox.Length, 0.01);
+        Assert.Equal(10, bbox.Length, 0.01);
+        Assert.Equal(5, bbox.Width, 0.01);
     }
 
     [Fact]