A 58-year-old man presents with progressive jaundice, pale stools, and dark urine for 3 weeks. Clinical examination reveals a palpable gallbladder and hepatomegaly. Serum bilirubin is 8.2 mg/dL (predominantly direct), ALP 320 U/L, and ALT 95 U/L. Abdominal ultrasound shows dilated intrahepatic and extrahepatic bile ducts with a hypoechoic lesion in the pancreatic head. Which investigation is most appropriate for definitive diagnosis and staging before planning surgical intervention?