A 7-year-old boy from Delhi presents with a 2-month history of progressive pallor, bone pain, and recurrent infections. On examination, he has hepatosplenomegaly and generalized lymphadenopathy. Complete blood count shows: Hemoglobin 6.8 g/dL, WBC 120,000/μL with 85% blasts, platelets 25,000/μL. Bone marrow aspirate reveals >90% blasts with fine chromatin, prominent nucleoli, and abundant cytoplasm with vacuoles. Flow cytometry shows blasts positive for CD10, CD19, CD20 (partial), HLA-DR, and TdT, negative for MPO and CD33. Cytogenetics reveals t(12;21)(p13;q22). What is the most likely diagnosis?