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/Pipeline/PackingPipeline.cs

@@ -0,0 +1,39 @@
+namespace CutList.Core.Nesting.Pipeline
+{
+    /// <summary>
+    /// Executes a sequence of packing steps to produce a final result.
+    /// Provides a composable, testable approach to bin packing algorithms.
+    /// </summary>
+    public class PackingPipeline
+    {
+        private readonly List<IPackingStep> _steps = new();
+
+        /// <summary>
+        /// Adds a step to the pipeline.
+        /// </summary>
+        /// <param name="step">The step to add.</param>
+        /// <returns>This pipeline for fluent chaining.</returns>
+        public PackingPipeline AddStep(IPackingStep step)
+        {
+            _steps.Add(step ?? throw new ArgumentNullException(nameof(step)));
+            return this;
+        }
+
+        /// <summary>
+        /// Executes all steps in sequence and returns the result.
+        /// </summary>
+        /// <param name="request">The packing request to process.</param>
+        /// <returns>The packing result.</returns>
+        public PackResult Execute(PackingRequest request)
+        {
+            var context = new PackingContext(request);
+
+            foreach (var step in _steps)
+            {
+                step.Execute(context);
+            }
+
+            return context.ToResult();
+        }
+    }
+}