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
+            };
+        }
+    }
+}