Your use the namespace "Fuel\Core", which is reserved for the framework components itself. Your App by default lives in the global namespace, so no namespace definition is needed.
Second, if your controller lives in ./classes/controller/welcome/welcome.php, the corresponding class name must be "Controller_Welcome_Welcome". The cascading file system works by taking the path (from classes, ucfirst the folder names, and replace the directory separator by an underscore.
If you want to namespace you App, you need to change the controller prefix in your config from "Controller_" to "\\Controller". You can then do
namespace Controller\Welcome; class Welcome extends \Controller {}
or
namespace Controller; class Welcome_Welcome extends \Controller {}