feat(console): add --engine CLI flag to select nest engine

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

OpenNest.Console/Program.cs

@@ -121,6 +121,9 @@ static class NestConsole
                 case "--autonest":
                     o.AutoNest = true;
                     break;
+                case "--engine" when i + 1 < args.Length:
+                    NestEngineRegistry.ActiveEngineName = args[++i];
+                    break;
                 case "--help":
                 case "-h":
                     PrintUsage();