Managers of $9.3 billion Artisan Mid Cap Fund emphasize that they are long-term managers. They refuse to dance in step with every short-term market do-si-do. But they don’t want to let a good thing pass them by either. In August’s broad market sell-off, they added to stocks they liked as valuations became too sweet to ignore. The fund added to NXP Semiconductors (NXPI), in which it had opened its position in the fourth quarter.