Function retry_sleep_under_total_deadline 

pub fn retry_sleep_under_total_deadline(
    total_deadline: Option<Duration>,
    elapsed: Duration,
    delay: Duration,
) -> Option<Duration>

Returns the sleep duration to use for a retry under an optional cross-attempt total deadline (REL-01), or None when the deadline has been exhausted and the retry must be aborted. When the deadline is set and the full delay would cross it, the sleep is clamped to the remaining slice so the loop never sleeps past the deadline. Pure in elapsed so tests can pin the clock; the production wrapper is retry_sleep_within_total.

• total_deadline == None: the full delay (unbounded, the v1 default).
• elapsed >= total_deadline: None (abort the retry).
• otherwise: min(delay, total_deadline - elapsed).