refactor: migrate WinForms callsites to NestEngineRegistry

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

OpenNest/Controls/PlateView.cs

@@ -833,7 +833,7 @@ namespace OpenNest.Controls
 
             try
             {
-                var engine = new NestEngine(Plate);
+                var engine = NestEngineRegistry.Create(Plate);
                 var parts = await Task.Run(() =>
                     engine.Fill(groupParts, workArea, progress, cts.Token));