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/FirstFitDecreasingStep.cs

@@ -0,0 +1,34 @@
+namespace CutList.Core.Nesting.Pipeline
+{
+    /// <summary>
+    /// Implements the First-Fit Decreasing (FFD) bin packing algorithm.
+    /// Assumes items are already sorted by length descending.
+    /// Places each item in the first bin that has enough space.
+    /// </summary>
+    public class FirstFitDecreasingStep : IPackingStep
+    {
+        public void Execute(PackingContext context)
+        {
+            while (context.RemainingItems.Count > 0 && context.CanAddMoreBins())
+            {
+                var bin = context.CreateBin();
+                FillBin(bin, context.RemainingItems);
+                context.Bins.Add(bin);
+            }
+        }
+
+        private static void FillBin(Bin bin, List<BinItem> remainingItems)
+        {
+            for (int i = 0; i < remainingItems.Count; i++)
+            {
+                var item = remainingItems[i];
+                if (bin.RemainingLength >= item.Length)
+                {
+                    bin.AddItem(item);
+                    remainingItems.RemoveAt(i);
+                    i--;
+                }
+            }
+        }
+    }
+}