v0.32.1 — Sampling-expiry churn under lock contention and analytics cohort crash

• Stop queued work from losing its turn to sample expiry under lock
  contention. Waiters sample memory before taking the queue's registry lock;
  with about 40 waiters the lock wait alone exceeded the two-second freshness
  limit, so admission denied with "sampling" over and over while other
  supervisors were publishing fresh samples. v0.32.0 recorded 21,445 such
  denials, and short jobs waited about an hour on October 19. After taking the
  lock, a waiter now adopts a newer published sample of the same configuration
  that is still under two seconds old. It only reads the cache and never probes
  under the lock. A failed measurement is never replaced, and red pressure,
  headroom, slots and the launch-time pressure recheck are unchanged.
  memcap analytics today reports how often this happens as "refreshed under
  lock".
• Fix memcap analytics today crashing when a retained job had no build, for
  example a pending job whose queued row was evicted. Such jobs now report
  their build from any surviving row, or as "unknown".