feat: add IFillComparer hooks to NestEngineBase

Add virtual comparer, direction, and angle-building hooks to NestEngineBase
so subclasses can override scoring and direction policy. Rewire IsBetterFill
to delegate to the comparer instead of calling FillScore directly.

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

OpenNest.Engine/NestEngineBase.cs

@@ -1,4 +1,6 @@
+using OpenNest.Engine;
 using OpenNest.Engine.Fill;
+using OpenNest.Engine.Strategies;
 using OpenNest.Geometry;
 using System;
 using System.Collections.Generic;
@@ -31,6 +33,25 @@ namespace OpenNest
 
         public abstract string Description { get; }
 
+        // --- Engine policy ---
+
+        private IFillComparer _comparer;
+
+        protected IFillComparer Comparer => _comparer ??= CreateComparer();
+
+        protected virtual IFillComparer CreateComparer() => new DefaultFillComparer();
+
+        public virtual NestDirection? PreferredDirection => null;
+
+        public virtual List<double> BuildAngles(NestItem item, double bestRotation, Box workArea)
+        {
+            return new List<double> { bestRotation, bestRotation + OpenNest.Math.Angle.HalfPI };
+        }
+
+        protected virtual void RecordProductiveAngles(List<AngleResult> angleResults) { }
+
+        protected FillPolicy BuildPolicy() => new FillPolicy(Comparer, PreferredDirection);
+
         // --- Virtual methods (side-effect-free, return parts) ---
 
         public virtual List<Part> Fill(NestItem item, Box workArea,
@@ -255,15 +276,7 @@ namespace OpenNest
         }
 
         protected bool IsBetterFill(List<Part> candidate, List<Part> current, Box workArea)
-        {
+            => Comparer.IsBetter(candidate, current, 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);
-        }
 
         protected bool IsBetterValidFill(List<Part> candidate, List<Part> current, Box workArea)
         {