Pull the `_child.py` `__main__` block body out into
a callable `_actor_child_main()` so alternate spawn
backends can bootstrap a subactor without going
through the CLI entrypoint.
Deats,
- new `_actor_child_main(uid, loglevel, parent_addr,
infect_asyncio, spawn_method='trio')` holds the
full child-side runtime startup previously inlined
under `if __name__ == '__main__':`
- `__main__` block reduces to arg-parsing + a call
into the new func
- add `"subint"` to the `_runtime.py` spawn-method
check so a child accepts `SpawnSpec` from that
(future) backend; inert str-compare w/o it
(this patch was generated in some part by [`claude-code`][claude-code-gh])
[claude-code-gh]: https://github.com/anthropics/claude-code
(cherry picked from commit b8f243e98d)
(factored: kept only the `_child.py`/`_runtime.py` entry-extraction parts of
"Impl min-viable `subint` spawn backend (B.2)"; dropped
tractor/spawn/_subint.py + subint prompt-io logs)
Gud Boi
8726323170