A 34-week gestation female infant born to a 28-year-old mother with gestational diabetes is admitted to the neonatal unit at 4 hours of life with tachypnea (respiratory rate 68/min), mild intercostal retractions, and grunting. Chest X-ray shows hyperinflation with prominent perihilar streaking and fluid in the fissures. Arterial blood gas on room air: pH 7.34, PaCO₂ 38 mmHg, PaO₂ 82 mmHg. The infant received antenatal corticosteroids. What is the most likely diagnosis?