A 3-year-old boy with cyanotic heart disease presents with a history of squatting posture during play and episodes of cyanosis. Clinical examination reveals a single S2 and a systolic ejection murmur at the left upper sternal border. Chest X-ray shows a 'boot-shaped' heart with decreased pulmonary vascular markings. Which investigation is most appropriate to confirm the diagnosis and assess the anatomical details for surgical planning?