Function prefill_varlen_enabled 

pub fn prefill_varlen_enabled() -> bool

VARLEN (ragged) batched prefill enabled? (--prefill-varlen-batch, legacy ATLAS_PREFILL_VARLEN=1; default OFF).

SSOT for the admission predicate (check_kernel_batched_eligible), the batched-attention layer’s chunk-0 guard, and the scheduler’s prefill wave planner. Those must agree: if admission accepts a batch the layer then rejects, the bail happens mid-Phase-A with streams already mutated, and the per-stream fallback re-runs setup on dirty state.