Move geometry primitives to OpenNest.Geometry namespace

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

OpenNest.Core/Plate.cs

@@ -2,6 +2,7 @@
 using System.Collections.Generic;
 using System.Linq;
 using OpenNest.Collections;
+using OpenNest.Geometry;
 using OpenNest.Math;
 
 namespace OpenNest