Function tier_level 

pub fn tier_level(
    priorities: &[i64],
    index: usize,
    smaller_is_urgent: bool,
) -> Option<u8>

phase-509 P7 (RFC-0108 D8) – which level an executor is, and whether every level of the image is alive: what a supervisor that owns a route per level or feeds a watchdog reads, and what a board’s tier runner states (the level count, and each tier’s level by tier_level_by). Plain atomics in nros-node, so ungated. tier_level_by over a slice of priorities.