Retrieves the shared SUIT cerebellar surface mesh used for vertex-based
cerebellar atlas rendering. The mesh contains 28,935 original surface
vertices plus 1,078 additional vertices forming a cap over the
peduncular surface (where the cerebellum meets the brainstem). The cap
vertices (indices 28,935--30,012) are duplicates of the boundary loop
and a centroid, designed to render as an opaque grey wall when not
assigned to any atlas region.