Constant FLUSHER_BARRIER_TIMEOUT 

pub(super) const FLUSHER_BARRIER_TIMEOUT: Duration;

Budget for the fallback flush barrier. Deliberately the REMAINDER of FLUSHER_DRAIN_TIMEOUT after FLUSHER_EOF_GRACE, so the two waits split one budget instead of each claiming a full one — the worst-case exit delay is unchanged from before the barrier existed, rather than doubled (muxreview P2 on PR #3261).