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/IEngine.cs

@@ -1,7 +1,16 @@
-namespace CutList.Core.Nesting
+namespace CutList.Core.Nesting
 {
+    /// <summary>
+    /// Interface for bin packing engines.
+    /// Engines are stateless - all configuration is passed via PackingRequest.
+    /// </summary>
     public interface IEngine
     {
-        Result Pack(List<BinItem> items);
+        /// <summary>
+        /// Packs items into bins according to the request configuration.
+        /// </summary>
+        /// <param name="request">The packing configuration and items.</param>
+        /// <returns>The packing result with bins and unused items.</returns>
+        PackResult Pack(PackingRequest request);
     }
-}
+}