chore: sort using directives

Auto-formatter reordering of using statements across the solution.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

OpenNest.Engine/RectanglePacking/PackFirstFitDecreasing.cs

@@ -1,6 +1,6 @@
-using System.Collections.Generic;
+using OpenNest.Geometry;
+using System.Collections.Generic;
 using System.Linq;
-using OpenNest.Geometry;
 
 namespace OpenNest.RectanglePacking
 {