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/PackingStrategy.cs

@@ -0,0 +1,31 @@
+namespace CutList.Core.Nesting
+{
+    /// <summary>
+    /// Specifies the bin packing strategy/algorithm to use.
+    /// </summary>
+    public enum PackingStrategy
+    {
+        /// <summary>
+        /// First-Fit Decreasing with optimization passes.
+        /// Sorts items by length descending, then packs using first-fit.
+        /// Includes optimization to improve packing by swapping items.
+        /// Best for general-purpose use cases.
+        /// </summary>
+        AdvancedFit,
+
+        /// <summary>
+        /// Best-Fit Decreasing algorithm.
+        /// Sorts items by length descending, then places each item in the bin
+        /// with the least remaining space that can still fit it.
+        /// Simpler but can be effective for certain distributions.
+        /// </summary>
+        BestFit,
+
+        /// <summary>
+        /// Exhaustive search that tries all possible combinations.
+        /// Guarantees optimal solution but has exponential time complexity.
+        /// Automatically falls back to AdvancedFit for more than 15 items.
+        /// </summary>
+        Exhaustive
+    }
+}