Add factory pattern for engine creation

Replace hard-coded engine instantiation with factory pattern
to enable dependency injection and improve testability.

Changes:
- IEngineFactory: Interface for creating engines
- EngineFactory: Default implementation using AdvancedFitEngine
- MultiBinEngine: Now accepts IEngineFactory via constructor

This eliminates the hard-coded dependency on AdvancedFitEngine
and allows for easy swapping of engine implementations through
configuration or dependency injection.

Benefits:
- Dependency inversion principle satisfied
- Engines can be mocked in tests
- Engine selection can be configured externally
- Single responsibility for engine creation

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

SawCut/Nesting/EngineFactory.cs

@@ -0,0 +1,19 @@
+namespace SawCut.Nesting
+{
+    /// <summary>
+    /// Default implementation of IEngineFactory that creates AdvancedFitEngine instances.
+    /// Can be extended to support different engine types based on configuration.
+    /// </summary>
+    public class EngineFactory : IEngineFactory
+    {
+        public IEngine CreateEngine(double stockLength, double spacing, int maxBinCount)
+        {
+            return new AdvancedFitEngine
+            {
+                StockLength = stockLength,
+                Spacing = spacing,
+                MaxBinCount = maxBinCount
+            };
+        }
+    }
+}

SawCut/Nesting/IEngineFactory.cs

@@ -0,0 +1,18 @@
+namespace SawCut.Nesting
+{
+    /// <summary>
+    /// Factory interface for creating bin packing engines.
+    /// Allows for dependency injection and testing without hard-coded engine types.
+    /// </summary>
+    public interface IEngineFactory
+    {
+        /// <summary>
+        /// Creates a configured engine instance for bin packing.
+        /// </summary>
+        /// <param name="stockLength">The length of stock bins</param>
+        /// <param name="spacing">The spacing/kerf between items</param>
+        /// <param name="maxBinCount">Maximum number of bins to create</param>
+        /// <returns>A configured IEngine instance</returns>
+        IEngine CreateEngine(double stockLength, double spacing, int maxBinCount);
+    }
+}

SawCut/Nesting/MultiBinEngine.cs

@@ -6,6 +6,17 @@ namespace SawCut.Nesting
 {
     public class MultiBinEngine : IEngine
     {
+        private readonly IEngineFactory _engineFactory;
+
+        public MultiBinEngine() : this(new EngineFactory())
+        {
+        }
+
+        public MultiBinEngine(IEngineFactory engineFactory)
+        {
+            _engineFactory = engineFactory ?? throw new ArgumentNullException(nameof(engineFactory));
+        }
+
         public List<MultiBin> Bins { get; set; }
 
         public double Spacing { get; set; }
@@ -23,11 +34,8 @@ namespace SawCut.Nesting
 
             foreach (var bin in bins)
             {
-                var e = new AdvancedFitEngine();
-                e.MaxBinCount = bin.Quantity;
-                e.StockLength = bin.Length;
-                e.Spacing = Spacing;
-                var r = e.Pack(remainingItems);
+                var engine = _engineFactory.CreateEngine(bin.Length, Spacing, bin.Quantity);
+                var r = engine.Pack(remainingItems);
 
                 result.AddBins(r.Bins);
                 remainingItems = r.ItemsNotUsed.ToList();