Function register_builtin_altrep_classes 

pub(crate) fn register_builtin_altrep_classes()

Eagerly register all built-in ALTREP classes.

Must be called during R_init so that R can find these classes when unserializing (readRDS) in a fresh session. Without this, the lazy OnceLock registration means classes don’t exist until first use — too late for readRDS.