Function set_prefill_varlen_from_cli 

pub fn set_prefill_varlen_from_cli(enabled: bool) -> bool

2026-09-25: Publish the command line’s --prefill-varlen-batch decision. Returns the value in force, which differs from enabled when the cell was already resolved; the command line then did not take effect. The serve calls this only when the flag is given, so without it the METRALE_PREFILL_VARLEN fallback applies.