Comment/docstring updates: `subint_forkserver` is a clean
`NotImplementedError` stub — not an alias to variant-1
(`main_thread_forkserver`). Key reserved in-place (not aliased) so
the subint-hosted-child impl can flip without API churn once
jcrist/msgspec#1026 unblocks PEP 684 subints.
(this commit msg was generated in some part by [`claude-code`][claude-code-gh])
[claude-code-gh]: https://github.com/anthropics/claude-code
(cherry picked from commit 7d1e4462d4)
Gud Boi
c6a4782f57