A 28-year-old woman of African descent presents with jaundice, dark urine, and right upper quadrant pain. She reports a recent upper respiratory tract infection 2 weeks ago. On examination, she is icteric with splenomegaly. Laboratory investigations reveal: Hemoglobin 7.2 g/dL, reticulocyte count 12%, total bilirubin 4.8 mg/dL (unconjugated 4.2 mg/dL), LDH 680 U/L, haptoglobin <10 mg/dL. Peripheral blood smear shows polychromasia and spherocytes. Direct antiglobulin test (DAT/Coombs) is negative. What is the most likely diagnosis?