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/RectanglePacking/RectFill.cs

@@ -0,0 +1,44 @@
+using OpenNest.Geometry;
+
+namespace OpenNest.RectanglePacking
+{
+    internal static class RectFill
+    {
+        public static void FillBest(Bin bin, Item item, int maxCount = int.MaxValue)
+        {
+            var spiralBin = bin.Clone() as Bin;
+            var spiral = new FillSpiral(spiralBin);
+            spiral.Fill(item, maxCount);
+
+            // Recursively fill the center remnant of the spiral
+            if (spiralBin.Items.Count > 0 && spiral.CenterRemnant != null)
+            {
+                var center = spiral.CenterRemnant;
+                var fitsNormal = item.Length <= center.Length && item.Width <= center.Width;
+                var fitsRotated = item.Width <= center.Length && item.Length <= center.Width;
+
+                if (fitsNormal || fitsRotated)
+                {
+                    var remaining = maxCount - spiralBin.Items.Count;
+                    FillBest(center.Location, center.Size, spiralBin, item, remaining);
+                }
+            }
+
+            var bestFitBin = bin.Clone() as Bin;
+            new FillBestFit(bestFitBin).Fill(item, maxCount);
+
+            var winner = spiralBin.Items.Count >= bestFitBin.Items.Count ? spiralBin : bestFitBin;
+            bin.Items.AddRange(winner.Items);
+        }
+
+        public static void FillBest(Vector location, Size size, Bin target, Item item, int maxCount)
+        {
+            if (size.Width <= 0 || size.Length <= 0 || maxCount <= 0)
+                return;
+
+            var bin = new Bin { Location = location, Size = size };
+            FillBest(bin, item, maxCount);
+            target.Items.AddRange(bin.Items);
+        }
+    }
+}