When the 429 retry deadline stopped a lineage, each live descendant was
interrupted one after another with its own 10s wait, and every rescan
that found a new descendant started a fresh wait. Several silent
descendants pushed the unconfirmed stop bound back by 10s each.
Interrupt descendants concurrently and share one interrupt deadline,
set on the first descendant scan, across later rescans in both the
stalled-turn and lineage settle paths.