A 58-year-old man presents with painless progressive jaundice for 3 weeks. He has lost 5 kg in weight over 2 months. On examination, Courvoisier's sign is positive. Serum bilirubin is 8.2 mg/dL (direct 6.8), alkaline phosphatase 420 IU/L, and transaminases are mildly elevated. Abdominal ultrasound shows dilated intrahepatic bile ducts, a dilated common bile duct (12 mm), and a hypoechoic lesion in the pancreatic head. What is the most appropriate next investigation to confirm the diagnosis and guide surgical planning?