Drop subint-family gate from `main_thread_forkserver`

`main_thread_forkserver` doesn't actually need py3.14
`concurrent.interpreters` (PEP 734) — it forks from a
non-trio worker thread and runs `_trio_main` in the child,
same shape as `trio_proc`. The previous `_has_subints`
gate + subint-family `case` arm were a copy-paste error.

In `tractor.spawn._main_thread_forkserver`,
- drop the `_has_subints` import + the `RuntimeError`
  raise in `main_thread_forkserver_proc()`.
- drop the now-unused `import sys` (only used by the
  prior error msg).

In `tractor.spawn._spawn.try_set_start_method()`,
- pull `'main_thread_forkserver'` out of the subint-
  family arm (which still gates on `_has_subints`).
- merge it into the `'trio'` arm — both set `_ctx = None`
  bc neither needs an `mp.context`.

(this commit msg was generated in some part by [`claude-code`][claude-code-gh])
[claude-code-gh]: https://github.com/anthropics/claude-code

tractor/spawn/_main_thread_forkserver.py

@@ -346,7 +346,6 @@ from __future__ import annotations
 import errno
 import os
 import signal
-import sys
 import threading
 from functools import partial
 from typing import (
@@ -370,7 +369,6 @@ from ._spawn import (
     cancel_on_completion,
     soft_kill,
 )
-from ._subint import _has_subints
 
 if TYPE_CHECKING:
     from tractor.discovery._addr import UnwrappedAddress
@@ -832,13 +830,6 @@ async def main_thread_forkserver_proc(
     thread instead of `trio.lowlevel.open_process()`.
 
     '''
-    if not _has_subints:
-        raise RuntimeError(
-            f'The {"main_thread_forkserver"!r} spawn backend '
-            f'requires Python 3.14+.\n'
-            f'Current runtime: {sys.version}'
-        )
-
     # Backend-scoped config pulled from `proc_kwargs`. Using
     # `proc_kwargs` (vs a first-class kwarg on this function)
     # matches how other backends expose per-spawn tuning

tractor/spawn/_spawn.py

@@ -135,13 +135,15 @@ def try_set_start_method(
         case 'mp_spawn':
             _ctx = mp.get_context('spawn')
 
-        case 'trio':
+        case (
+            'trio'
+            | 'main_thread_forkserver'
+        ):
             _ctx = None
 
         case (
             'subint'
             | 'subint_fork'
-            | 'main_thread_forkserver'
             | 'subint_forkserver'
         ):
             # All subint-family backends need no `mp.context`;