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/Splitting/DrawingSplitterTests.cs

@@ -161,11 +161,11 @@ public class DrawingSplitterTests
         var bb2 = results[1].Program.BoundingBox();
 
         // Piece lengths should sum to original length
-        Assert.Equal(100.0, bb1.Width + bb2.Width, 1);
+        Assert.Equal(100.0, bb1.Length + bb2.Length, 1);
 
         // Both pieces should have the same width as the original
-        Assert.Equal(100.0, bb1.Length, 1);
-        Assert.Equal(100.0, bb2.Length, 1);
+        Assert.Equal(100.0, bb1.Width, 1);
+        Assert.Equal(100.0, bb2.Width, 1);
     }
 
     [Fact]
@@ -183,11 +183,11 @@ public class DrawingSplitterTests
         var bb2 = results[1].Program.BoundingBox();
 
         // Piece widths should sum to original width
-        Assert.Equal(100.0, bb1.Length + bb2.Length, 1);
+        Assert.Equal(100.0, bb1.Width + bb2.Width, 1);
 
         // Both pieces should have the same length as the original
-        Assert.Equal(100.0, bb1.Width, 1);
-        Assert.Equal(100.0, bb2.Width, 1);
+        Assert.Equal(100.0, bb1.Length, 1);
+        Assert.Equal(100.0, bb2.Length, 1);
     }
 
     [Fact]
@@ -287,8 +287,8 @@ public class DrawingSplitterTests
         var bb2 = results[1].Program.BoundingBox();
 
         // Left piece should be 30 long, right piece should be 70 long
-        Assert.Equal(30.0, bb1.Width, 1);
-        Assert.Equal(70.0, bb2.Width, 1);
+        Assert.Equal(30.0, bb1.Length, 1);
+        Assert.Equal(70.0, bb2.Length, 1);
     }
 
     [Fact]