A 4-year-old boy presents with a 3-week history of progressive pallor, easy bruising, and recurrent fever. His mother reports he has been reluctant to play and complains of bone pain. On examination, he is pale, has petechiae on his lower limbs, hepatomegaly (3 cm below costal margin), and splenomegaly (2 cm below costal margin). Laboratory investigations reveal: Hemoglobin 7.2 g/dL, WBC 85,000/μL (with 70% blasts), platelets 35,000/μL. Bone marrow aspiration shows >90% blasts with high nuclear-to-cytoplasmic ratio and fine chromatin. Flow cytometry is positive for CD19, CD10, and TdT. What is the most likely diagnosis?