A 26-year-old woman with mild persistent asthma uses low-dose inhaled budesonide daily and albuterol as needed. Over the past 3 months, she has had increasing daytime symptoms 4-5 days per week and nighttime awakenings twice per month. Spirometry shows FEV₁ 80% predicted. According to current stepwise asthma guidelines, what is the best next step in controller therapy?