A child could pass the early `.start_actor()` guard, miss the
`.cancel()` child snapshot and register afterward. Its monitor
inherited a reap request without runtime cancellation and could wait
forever.
- publish child/reap events before sampling `_cancel_called`
- make MP abort before `proc.start()` when cancellation won
- kill a Trio child opened after cancellation won registration
- reject starts begun after nursery cancellation is already visible
- add deterministic registration and MP no-start regressions
- drop the touched Trio backend's stale `get_runtime_vars` import
Prompt-IO: ai/prompt-io/opencode/20260821T040803Z_3c1bbe73_prompt_io.md
(this patch was generated in some part by `opencode` using
`gpt-5.6-sol` (`openai`))
Gud Boi