feat(engine): add RectBestFitStrategy adapter

Wraps FillBestFit rectangle packer in an IFillStrategy so the rect
best-fit phase participates in the pluggable pipeline.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

OpenNest.Engine/Strategies/RectBestFitStrategy.cs

@@ -0,0 +1,23 @@
+using System.Collections.Generic;
+using OpenNest.RectanglePacking;
+
+namespace OpenNest
+{
+    public class RectBestFitStrategy : IFillStrategy
+    {
+        public string Name => "RectBestFit";
+        public NestPhase Phase => NestPhase.RectBestFit;
+        public int Order => 200;
+
+        public List<Part> Fill(FillContext context)
+        {
+            var binItem = BinConverter.ToItem(context.Item, context.Plate.PartSpacing);
+            var bin = BinConverter.CreateBin(context.WorkArea, context.Plate.PartSpacing);
+
+            var engine = new FillBestFit(bin);
+            engine.Fill(binItem);
+
+            return BinConverter.ToParts(bin, new List<NestItem> { context.Item });
+        }
+    }
+}