A 10-year-old boy has had persistent headaches for the past 3 months. On physical examination, he is afebrile. He has an ataxic gait and dysdiadochokinesia. CT scan of the head shows a 4-cm cystic mass in the right cerebellar hemisphere. Cerebral lateral ventricles are enlarged. A lumbar puncture is done. The CSF protein concentration is elevated, but the glucose level is normal. Neurosurgery is performed, and the mass is removed and sectioned. On gross examination, the mass is a cyst filled with gelatinous material. The cyst has a thin wall and a 1-cm mural nodule. Microscopically, the mass is composed of cells that stain positive for glial fibrillary acidic protein (GFAP) and have long, hair-like processes. What is the most likely diagnosis?