chore: Remove unused using statements

Cleanup of unnecessary imports across the codebase.

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

CutList.Core/Nesting/MultiBinEngine.cs

@@ -1,8 +1,4 @@
-using System;
-using System.Collections.Generic;
-using System.Linq;
-
-namespace CutList.Core.Nesting
+namespace CutList.Core.Nesting
 {
     public class MultiBinEngine : IEngine
     {