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/IPackingStep.cs

@@ -0,0 +1,15 @@
+namespace CutList.Core.Nesting.Pipeline
+{
+    /// <summary>
+    /// Represents a single step in the packing pipeline.
+    /// Each step modifies the PackingContext to progress toward a final result.
+    /// </summary>
+    public interface IPackingStep
+    {
+        /// <summary>
+        /// Executes this step, modifying the context as needed.
+        /// </summary>
+        /// <param name="context">The mutable packing context.</param>
+        void Execute(PackingContext context);
+    }
+}