Function check_tier_executor_backing 

pub fn check_tier_executor_backing(
    n_tiers: usize,
    backing: &[TierExecutorBackingSlot],
) -> Result<(), TierBackingShort>

issue 1571 — the ONE refusal every board’s run_tiers makes before it opens anything: n_tiers tiers run, one on the boot executor, so n_tiers - 1 must each find a slot. Checked up front rather than at each spawn, so a short block never leaves a half-spawned tier chain.