A 28-year-old woman presents to the orthopedic clinic with a 6-month history of progressive pain and swelling over the right knee. On examination, there is localized warmth and mild effusion. Plain radiographs reveal a lytic lesion in the distal femur, eccentrically located in the metaphyseal-epiphyseal region, extending to within 1 cm of the articular surface. The lesion shows a thin sclerotic margin with a "soap bubble" appearance. MRI confirms a well-circumscribed mass with mixed signal intensity and hemosiderin deposits. What is the most likely diagnosis?