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/IEngineFactory.cs

@@ -7,12 +7,10 @@ namespace CutList.Core.Nesting
     public interface IEngineFactory
     {
         /// <summary>
-        /// Creates a configured engine instance for bin packing.
+        /// Creates an engine instance for the specified packing strategy.
         /// </summary>
-        /// <param name="stockLength">The length of stock bins</param>
-        /// <param name="spacing">The spacing/kerf between items</param>
-        /// <param name="maxBinCount">Maximum number of bins to create</param>
-        /// <returns>A configured IEngine instance</returns>
-        IEngine CreateEngine(double stockLength, double spacing, int maxBinCount);
+        /// <param name="strategy">The packing strategy to use.</param>
+        /// <returns>A configured IEngine instance.</returns>
+        IEngine CreateEngine(PackingStrategy strategy = PackingStrategy.AdvancedFit);
     }
 }