A 42-year-old woman presents with fatigue and breathlessness. FBC shows Hb 80 g/L, MCV 85 fL, WCC 10 x10^9/L, Platelets 250 x10^9/L. Reticulocyte count is increased (150 x10^9/L). Blood film shows spherocytes. Bilirubin is raised (40 µmol/L, mostly unconjugated). LDH is elevated. Direct Coombs test (DAT) is positive. What is the most likely diagnosis?