refactor: Redesign nesting engines with pipeline pattern and add exhaustive search

- Rename Result to PackResult to avoid confusion with Result<T>
- Add PackingRequest as immutable configuration replacing mutable engine state
- Add PackingStrategy enum (AdvancedFit, BestFit, Exhaustive)
- Implement pipeline pattern for composable packing steps
- Rewrite AdvancedFitEngine as stateless using pipeline
- Rewrite BestFitEngine as stateless
- Add ExhaustiveFitEngine with symmetry breaking for optimal solutions
  - Tries all bin assignments to find minimum bins
  - Falls back to AdvancedFit for >20 items
  - Configurable threshold via constructor
- Update IEngine/IEngineFactory interfaces for new pattern
- Add strategy parameter to MCP tools

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

CutList.Core/Nesting/PackResult.cs

@@ -0,0 +1,54 @@
+namespace CutList.Core.Nesting
+{
+    /// <summary>
+    /// Represents the result of a bin packing operation.
+    /// Contains the packed bins and any items that could not be placed.
+    /// </summary>
+    public class PackResult
+    {
+        private readonly List<BinItem> _itemsNotUsed;
+        private readonly List<Bin> _bins;
+
+        public PackResult()
+        {
+            _itemsNotUsed = new List<BinItem>();
+            _bins = new List<Bin>();
+        }
+
+        public PackResult(IEnumerable<Bin> bins, IEnumerable<BinItem> itemsNotUsed)
+        {
+            _bins = bins?.ToList() ?? new List<Bin>();
+            _itemsNotUsed = itemsNotUsed?.ToList() ?? new List<BinItem>();
+        }
+
+        /// <summary>
+        /// Items that could not be placed in any bin (e.g., too large for stock).
+        /// </summary>
+        public IReadOnlyList<BinItem> ItemsNotUsed => _itemsNotUsed;
+
+        /// <summary>
+        /// The bins containing packed items.
+        /// </summary>
+        public IReadOnlyList<Bin> Bins => _bins;
+
+        public void AddItemNotUsed(BinItem item)
+        {
+            _itemsNotUsed.Add(item);
+        }
+
+        public void AddItemsNotUsed(IEnumerable<BinItem> items)
+        {
+            _itemsNotUsed.AddRange(items);
+        }
+
+        public void AddBin(Bin bin)
+        {
+            _bins.Add(bin);
+        }
+
+        public void AddBins(IEnumerable<Bin> bins)
+        {
+            _bins.AddRange(bins);
+        }
+    }
+}