Child monitors register before their IPC handshake so cancellation
owns every started process, but the bare `None` portal arg obscured
that `Portal(chan)` replaces the provisional entry after connection.
Document that transition and name all `_register_child()` args in both
spawn backends. Replace the MP test's positional-only lambda with a
signature-accurate fake which asserts the provisional portal state.
Caught-during: review remediation
Found-via: `/run-tests` test_mp_late_registration_never_starts_process
Review: PR #481 (goodboy)
https://github.com/goodboy/tractor/pull/481#pullrequestreview-5012942328
Prompt-IO: ai/prompt-io/opencode/20260825T015742Z_e42ecb55_prompt_io.md
(this patch was generated in some part by `opencode` using `gpt-5.6-sol` (`openai`))
Gud Boi