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/IFillComparer.cs

@@ -0,0 +1,14 @@
+using OpenNest.Geometry;
+using System.Collections.Generic;
+
+namespace OpenNest.Engine
+{
+    /// <summary>
+    /// Determines whether a candidate fill result is better than the current best.
+    /// Implementations must be stateless and thread-safe.
+    /// </summary>
+    public interface IFillComparer
+    {
+        bool IsBetter(List<Part> candidate, List<Part> current, Box workArea);
+    }
+}