A 14-year-old boy from rural India presents with a 3-month history of progressive pain and swelling over the mid-shaft of his left femur. He reports fever on and off for the past 6 weeks and has lost 4 kg of body weight. On examination, the swelling is warm, tender, and fixed to underlying bone. Laboratory investigations show hemoglobin 10.2 g/dL, WBC 14,500/μL with left shift, and ESR 68 mm/hr. Plain radiographs reveal a mixed lytic and sclerotic lesion in the diaphysis with an onion-skin periosteal reaction and soft tissue mass. What is the most likely diagnosis?