A 28-year-old woman from Bangalore presents with a 6-month history of progressive right hip pain. Plain radiographs show a well-defined, eccentric lytic lesion in the femoral head with a thin sclerotic rim. MRI reveals the lesion is confined to the epiphysis with no cortical breakthrough. The lesion is hypointense on T1 and hyperintense on T2, with a characteristic "soap bubble" appearance on CT. Pulmonary imaging is normal. What is the most likely diagnosis?