A 7-year-old boy from Tamil Nadu presents with severe pallor, jaundice, and hepatosplenomegaly. His mother reports he has required blood transfusions every 4–6 weeks since age 2. Hemoglobin is 7.2 g/dL, MCV 62 fL, reticulocyte count 8%, and serum ferritin 2800 ng/mL. Peripheral blood smear shows hypochromic microcytic RBCs with target cells and nucleated RBCs. Hemoglobin electrophoresis reveals absent HbA, elevated HbF (80%), and HbA2 2%. What is the most likely diagnosis?