Reduce `_subint_forkserver.py` to its variant-2 placeholder shape:
- Add `subint_forkserver_proc` async stub raising `NotImplementedError`
with a redirect msg pointing at the working variant-1 backend
(`main_thread_forkserver`), jcrist/msgspec#1026 (upstream PEP 684
blocker), and #379 (subint umbrella).
- `tractor.spawn._spawn._methods['subint_forkserver']` now dispatches to
the stub instead of aliasing the variant-1 coroutine
— `--spawn-backend=subint_forkserver` errors cleanly.
- Drop now-dead module-scope: `ChildSigintMode`
/ `_DEFAULT_CHILD_SIGINT` defs, `_has_subints` try/except (replaced
with import from `._subint`), unused imports (`partial`, `Literal`,
`sys`, msgtypes/pretty_struct, `current_actor`,
`cancel_on_completion`/`soft_kill`, `_server` TYPE_CHECKING).
- Backward-compat re-exports of fork primitives kept until the follow-up
commit migrates external test imports.
- `tests/spawn/test_subint_forkserver.py::forkserver_spawn_method`
fixture: flip hardcoded `'subint_forkserver'`
→ `'main_thread_forkserver'` so the test still exercises the working
backend (full file rename comes in the test-import migration commit).
(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 5e83881f10)
Gud Boi
d7cbc71e84