Race the initial peer handshake against process exit during Trio
child bootstrap so a dead child cannot park its spawning task.
Restore and harden the design originally implemented in `3b0724eb`.
Deats,
- register peer events before provisional nursery publication
- raise `ActorFailure` with child status when process death wins
- prioritize observed death over a simultaneous handshake
- preserve waiter exceptions without `ExceptionGroup` wrapping
- log expected losing-waiter cancellation at debug level
- remove exact failed-startup peer events during backend cleanup
- cover controlled schedules and full Trio backend cleanup
Based-on: 3b0724eba8
Prompt-IO: ai/prompt-io/opencode/20260828T045119Z_fb6d81d3_prompt_io.md
(this patch was generated in some part by `opencode` using `gpt-5.6-sol` (`openai`))
Gud Boi