Function serial_verify_plan 

pub fn serial_verify_plan(
    pending: usize,
    step_drafts: usize,
    dflash: bool,
) -> (usize, SerialArm)

2026-09-25: How many of a sequence’s pending drafts the serial path keeps, and which arm verifies them. An MTP sequence can carry more pending drafts than this step’s width step_drafts (the ladder count after clamp_drafts_to_slot_capacity); the surplus is dropped so the verify never exceeds the slot. With dflash every pending draft is kept. Four or more kept drafts run the DFlash arm on a DFlash serve and the KN arm otherwise; fewer run K4, K3 or K2 by the kept count, each also bounded by step_drafts.