Rename SawCut library to CutList.Core

Rename the core library project from SawCut to CutList.Core for consistent
branding across the solution. This includes:
- Rename project folder and .csproj file
- Update namespace from SawCut to CutList.Core
- Update all using statements and project references

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

CutList.Core/Nesting/IEngineFactory.cs

@@ -0,0 +1,18 @@
+namespace CutList.Core.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);
+    }
+}