Function mtp_drafter_prefill_enabled 

pub fn mtp_drafter_prefill_enabled(levers: &ModelLevers) -> bool

2026-09-25: Whether the drafter prefill is on (levers.drafter.prefill, on unless METRALE_NO_MTP_DRAFTER_CONTEXT turns it off; policy in crate::drafter_context). The prefill builds the drafter’s KV over the whole prompt before the first propose. The drafter’s K/V depend only on its input pair, not on attention, so it runs the norms, fc, K/V projections, RoPE and the cache write, with no attention pass.