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/PackingRequest.cs

@@ -0,0 +1,51 @@
+namespace CutList.Core.Nesting
+{
+    /// <summary>
+    /// Immutable configuration object for a bin packing operation.
+    /// Replaces mutable engine state with a clean request/response pattern.
+    /// </summary>
+    public sealed class PackingRequest
+    {
+        /// <summary>
+        /// Creates a new packing request.
+        /// </summary>
+        /// <param name="items">The items to be packed.</param>
+        /// <param name="stockLength">The length of stock bins.</param>
+        /// <param name="spacing">The spacing/kerf between items (default 0).</param>
+        /// <param name="maxBinCount">Maximum number of bins to create (default unlimited).</param>
+        public PackingRequest(
+            IReadOnlyList<BinItem> items,
+            double stockLength,
+            double spacing = 0,
+            int maxBinCount = int.MaxValue)
+        {
+            if (stockLength <= 0)
+                throw new ArgumentException("Stock length must be greater than 0", nameof(stockLength));
+
+            Items = items ?? throw new ArgumentNullException(nameof(items));
+            StockLength = stockLength;
+            Spacing = spacing;
+            MaxBinCount = maxBinCount;
+        }
+
+        /// <summary>
+        /// The items to be packed into bins.
+        /// </summary>
+        public IReadOnlyList<BinItem> Items { get; }
+
+        /// <summary>
+        /// The length of each stock bin.
+        /// </summary>
+        public double StockLength { get; }
+
+        /// <summary>
+        /// The spacing/kerf between items (blade width).
+        /// </summary>
+        public double Spacing { get; }
+
+        /// <summary>
+        /// Maximum number of bins to create. Use int.MaxValue for unlimited.
+        /// </summary>
+        public int MaxBinCount { get; }
+    }
+}