Function set_prefill_varlen_from_cli 

pub fn set_prefill_varlen_from_cli(enabled: bool) -> bool

Publish the command line’s --prefill-varlen-batch decision. Returns the value IN FORCE, which differs from enabled when something already resolved the cell (then the command line did NOT take effect — the caller warns, mirroring gdn_flags::set_from_cli). Absent flag ⇒ never called ⇒ the documented ATLAS_PREFILL_VARLEN fallback stays reachable.