A 4-year-old boy from rural Maharashtra presents with a 3-week history of progressive pallor, easy bruising, and petechiae. His mother reports he has been febrile on and off and complains of bone pain. On examination: pallor, petechiae over lower limbs, hepatomegaly (3 cm), splenomegaly (2 cm), and generalized lymphadenopathy. CBC shows: Hb 7.2 g/dL, WBC 85,000/μL (with 70% blasts), platelets 45,000/μL. Bone marrow aspirate reveals >90% lymphoblasts with fine chromatin and scanty cytoplasm. Cytochemistry: MPO negative, PAS positive (diffuse). What is the most likely diagnosis?