fix: delegate Fill(groupParts) and PackArea to DefaultNestEngine

StripNestEngine only overrode Fill(NestItem), so ActionClone.Fill
and Pack operations fell through to the empty base class defaults.
Now all virtual methods delegate to DefaultNestEngine.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

OpenNest.Engine/StripNestEngine.cs

@@ -30,6 +30,26 @@ namespace OpenNest
             return inner.Fill(item, workArea, progress, token);
         }
 
+        /// <summary>
+        /// Group-parts fill delegates to DefaultNestEngine.
+        /// </summary>
+        public override List<Part> Fill(List<Part> groupParts, Box workArea,
+            IProgress<NestProgress> progress, CancellationToken token)
+        {
+            var inner = new DefaultNestEngine(Plate);
+            return inner.Fill(groupParts, workArea, progress, token);
+        }
+
+        /// <summary>
+        /// Pack delegates to DefaultNestEngine.
+        /// </summary>
+        public override List<Part> PackArea(Box box, List<NestItem> items,
+            IProgress<NestProgress> progress, CancellationToken token)
+        {
+            var inner = new DefaultNestEngine(Plate);
+            return inner.PackArea(box, items, progress, token);
+        }
+
         /// <summary>
         /// Selects the item that consumes the most plate area (bounding box area x quantity).
         /// Returns the index into the items list.