wakeChild property

Wakes a retained child with pending inbox mail by resuming it in its own session (gh-970). Hosts WITH the child-resume capability wire their executor's resume here: wakeChild = (id) => executor.resumeChild(id, childInboxWakePrompt) — the resumed run's warm wake drains the inbox as its first action, so the reminder (or sibling mail) that fired into a finished monitor is consumed and the recurrence continues. Null (the app host — no child-resume capability) keeps the sweep a no-op; the mail waits for the next task_send/steer.