A 35-year-old woman presents with a 6-month history of progressive pain and stiffness in the proximal humerus. On examination, there is mild limitation of shoulder abduction. Plain radiographs show a well-defined, sclerotic lesion in the proximal humeral metaphysis with a narrow zone of transition and a surrounding rim of sclerosis. There is no periosteal reaction or cortical destruction. CT imaging reveals internal calcification with a popcorn-like or chondroid appearance. MRI shows low-signal lesion with areas of high signal fat planes between the lesion and surrounding bone. Which is the most likely diagnosis?