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/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);
+    }
+}