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/Engine/NestProgressTests.cs

@@ -49,7 +49,7 @@ public class NestProgressTests
         var parts = new List<Part>
         {
             TestHelpers.MakePartAt(0, 0, 5),
-            TestHelpers.MakePartAt(10, 0, 5),
+            TestHelpers.MakePartAt(0, 10, 5),
         };
         var progress = new NestProgress { BestParts = parts };
         Assert.Equal(15, progress.NestedWidth, precision: 4);
@@ -61,7 +61,7 @@ public class NestProgressTests
         var parts = new List<Part>
         {
             TestHelpers.MakePartAt(0, 0, 5),
-            TestHelpers.MakePartAt(0, 10, 5),
+            TestHelpers.MakePartAt(10, 0, 5),
         };
         var progress = new NestProgress { BestParts = parts };
         Assert.Equal(15, progress.NestedLength, precision: 4);