A child that stalls under a configured delegation.child_timeout_seconds used to
learn about the budget only by dying, losing its whole context. The liveness
wait now queues a one-line "[delegation budget warning]" through the child's
steer channel once the idle window is 80% spent (delivered at the child's next
iteration boundary), so a slow-but-recoverable child can wrap up and return its
summary. The warning fires once per idle window and re-arms when progress
resets the window; a progressing child never sees it.
Part of #116001 (atom 2A). Semantics of child_timeout_seconds are unchanged.
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