Move math utilities to OpenNest.Math namespace

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

OpenNest.Core/Size.cs

@@ -49,7 +49,7 @@ namespace OpenNest
 
         public string ToString(int decimalPlaces)
         {
-            return string.Format("{0} x {1}", Math.Round(Height, decimalPlaces), Math.Round(Width, decimalPlaces));
+            return string.Format("{0} x {1}", System.Math.Round(Height, decimalPlaces), System.Math.Round(Width, decimalPlaces));
         }
     }
 }