A 3-day-old neonate born at term presents with severe cyanosis (SpO₂ 65% on room air) and respiratory distress. Clinical examination reveals a single loud S2 and an ejection systolic murmur at the left upper sternal border. Chest X-ray shows an 'egg-on-string' appearance with pulmonary oligemia. Which investigation is most appropriate to confirm the diagnosis and guide immediate management?