Function prefill_varlen_enabled 

pub fn prefill_varlen_enabled() -> bool

2026-09-25: Whether varlen (ragged) batched prefill is on: --prefill-varlen-batch, else METRALE_PREFILL_VARLEN (1 or true, any case); off when neither is set. The admission predicate (check_kernel_batched_eligible), the batched attention layer and the scheduler’s levers all read this one value.