feat: add IFillComparer interface and DefaultFillComparer

Extracts the fill result scoring contract into IFillComparer with a DefaultFillComparer implementation that preserves the existing count-then-density lexicographic ranking via FillScore.

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

OpenNest.Engine/Fill/DefaultFillComparer.cs

@@ -0,0 +1,23 @@
+using OpenNest.Geometry;
+using System.Collections.Generic;
+
+namespace OpenNest.Engine.Fill
+{
+    /// <summary>
+    /// Ranks fill results by count first, then density.
+    /// This is the original scoring logic used by DefaultNestEngine.
+    /// </summary>
+    public class DefaultFillComparer : IFillComparer
+    {
+        public bool IsBetter(List<Part> candidate, List<Part> current, Box workArea)
+        {
+            if (candidate == null || candidate.Count == 0)
+                return false;
+
+            if (current == null || current.Count == 0)
+                return true;
+
+            return FillScore.Compute(candidate, workArea) > FillScore.Compute(current, workArea);
+        }
+    }
+}