refactor: move GpuEvaluatorFactory to OpenNest.Gpu project

GPU factory logic belongs with the GPU implementation, not the UI.
Changed from internal to public and updated namespace to OpenNest.Gpu.

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

OpenNest/Actions/ActionClone.cs

@@ -4,6 +4,7 @@ using System.Linq;
 using System.Windows.Forms;
 using OpenNest.Controls;
 using OpenNest.Geometry;
+using OpenNest.Gpu;
 
 namespace OpenNest.Actions
 {

OpenNest/Actions/ActionFillArea.cs

@@ -1,6 +1,7 @@
 using System.ComponentModel;
 using System.Windows.Forms;
 using OpenNest.Controls;
+using OpenNest.Gpu;
 
 namespace OpenNest.Actions
 {