A 28-year-old man (B Rh-positive) undergoes elective open-heart surgery and receives 6 units of B Rh-positive packed RBCs intraoperatively. On postoperative day 2, he develops jaundice, dark urine, and a hemoglobin drop from 10 g/dL to 6.5 g/dL. Reticulocyte count is 8.5%, total bilirubin 4.2 mg/dL (direct 1.8 mg/dL), and LDH 680 U/L. Direct antiglobulin test (DAT/Coombs) is strongly positive. Blood bank reports that the transfused blood was correctly typed and cross-matched. What is the most likely diagnosis?