`._mark_child_reaped()` previously discarded the reap-request event
without checking that its completion-event peer existed. A one-sided
entry would silently lose process-reap synchronization.
Capture both pops and assert paired presence while allowing the valid
both-absent startup-failure path. Keep an unset request valid because
backend cancellation can reap immediately after registration.
Extend graceful and failed-cancel-ack runtime tests to require all
child and reap mappings empty before `to_actor.run()` returns.
Review: PR #481 (goodboy)
https://github.com/goodboy/tractor/pull/481#pullrequestreview-5012942328
Prompt-IO: ai/prompt-io/opencode/20260824T223614Z_88d538e3_prompt_io.md
(this patch was generated in some part by `opencode` using `gpt-5.6-sol` (`openai`))
Gud Boi