A 7-year-old boy from Tamil Nadu presents to the emergency department with acute onset severe pain in both hands and feet, swelling, and fever (38.5°C). His mother reports he was well until 6 hours ago. On examination, his hands and feet are warm, swollen, and exquisitely tender. His hemoglobin is 7.2 g/dL, reticulocyte count 8%, and peripheral blood smear shows sickle cells and target cells. X-ray of hands shows no fractures but soft tissue swelling. What is the most likely diagnosis?