A 28-year-old man presents with a 6-week history of progressive ataxia, nausea, and morning vomiting. Neurological examination reveals truncal ataxia and nystagmus. MRI brain shows a well-circumscribed, cystic mass in the midline cerebellum with a mural nodule. The cyst is hyperintense on T2 and FLAIR, with minimal surrounding edema. The mural nodule shows avid enhancement post-gadolinium. There is obstructive hydrocephalus at the level of the fourth ventricle. What is the most likely diagnosis?