A 16-year-old boy presents with a 3-month history of progressive pain and swelling over the distal femur. On examination, there is a firm, tender mass over the distal femur with warmth and overlying skin erythema. The patient reports a history of a minor knee injury 2 months ago. Plain radiographs show a mixed lytic and sclerotic lesion at the distal femoral metaphysis with cortical destruction and a Codman triangle. Serum alkaline phosphatase is elevated at 280 U/L (normal: 30–120). What is the most likely diagnosis?