chore: Remove unused using statements

Cleanup of unnecessary imports across the codebase.

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

CutList.Core/MultiBin.cs

@@ -1,6 +1,4 @@
-using System;
-
-namespace CutList.Core
+namespace CutList.Core
 {
     /// <summary>
     /// Represents a type of bin with quantity and priority.