Add `'subint'` spawn backend scaffold (#379)

Land the scaffolding for a future sub-interpreter (PEP 734
`concurrent.interpreters`) actor spawn backend per issue #379. The
spawn flow itself is not yet implemented; `subint_proc()` raises a
placeholder `NotImplementedError` pointing at the tracking issue —
this commit only wires up the registry, the py-version gate, and
the harness.

Deats,
- bump `pyproject.toml` `requires-python` to `>=3.12, <3.15` and
  list the `3.14` classifier — the new stdlib
  `concurrent.interpreters` module only ships on 3.14
- extend `SpawnMethodKey = Literal[..., 'subint']`
- `try_set_start_method('subint')` grows a new `match` arm that
  feature-detects the stdlib module and raises `RuntimeError` with
  a clear banner on py<3.14
- `_methods` registers the new `subint_proc()` via the same
  bottom-of-module late-import pattern used for `._trio` / `._mp`

Also,
- new `tractor/spawn/_subint.py` — top-level `try: from concurrent
  import interpreters` guards `_has_subints: bool`; `subint_proc()`
  signature mirrors `trio_proc`/`mp_proc` so the Phase B.2 impl can
  drop in without touching the registry
- re-add `import sys` to `_spawn.py` (needed for the py-version msg
  in the gate-error)
- `_testing.pytest.pytest_configure` wraps `try_set_start_method()`
  in a `pytest.UsageError` handler so `--spawn-backend=subint` on
  py<3.14 prints a clean banner instead of a traceback

(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 d318f1f8f4)
(MTF-only portion: kept tractor/spawn/_spawn.py tractor/spawn/_subint.py)

tractor/spawn/_spawn.py

@@ -22,6 +22,7 @@ over multiple backends.
 from __future__ import annotations
 import multiprocessing as mp
 import platform
+import sys
 from typing import (
     Any,
     Awaitable,
@@ -64,6 +65,7 @@ SpawnMethodKey = Literal[
     'trio',  # supported on all platforms
     'mp_spawn',
     'mp_forkserver',  # posix only
+    'subint',  # py3.14+ via `concurrent.interpreters` (PEP 734)
 ]
 _spawn_method: SpawnMethodKey = 'trio'
 
@@ -116,6 +118,17 @@ def try_set_start_method(
         case 'trio':
             _ctx = None
 
+        case 'subint':
+            # subints need no `mp.context`; feature-gate 3.14+
+            from ._subint import _has_subints
+            if not _has_subints:
+                raise RuntimeError(
+                    f'Spawn method {key!r} requires Python 3.14+ '
+                    f'(stdlib `concurrent.interpreters`, PEP 734).\n'
+                    f'Current runtime: {sys.version}'
+                )
+            _ctx = None
+
         case _:
             raise ValueError(
                 f'Spawn method `{key}` is invalid!\n'
@@ -465,6 +478,7 @@ async def new_proc(
 # `hard_kill`/`proc_waiter` from this module.
 from ._trio import trio_proc
 from ._mp import mp_proc
+from ._subint import subint_proc
 
 
 # proc spawning backend target map
@@ -472,4 +486,5 @@ _methods: dict[SpawnMethodKey, Callable] = {
     'trio': trio_proc,
     'mp_spawn': mp_proc,
     'mp_forkserver': mp_proc,
+    'subint': subint_proc,
 }

tractor/spawn/_subint.py

@@ -0,0 +1,100 @@
+# tractor: structured concurrent "actors".
+# Copyright 2018-eternity Tyler Goodlet.
+
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU Affero General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU Affero General Public License for more details.
+
+# You should have received a copy of the GNU Affero General Public License
+# along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+'''
+Sub-interpreter (`subint`) actor spawning backend.
+
+Spawns each sub-actor as a CPython PEP 734 sub-interpreter
+(`concurrent.interpreters.Interpreter`) — same-process state
+isolation with faster start-up than an OS subproc, while
+preserving tractor's IPC-based actor boundaries.
+
+Availability
+------------
+Requires Python 3.14+ for the stdlib `concurrent.interpreters`
+module. On older runtimes the module still imports (so the
+registry stays introspectable) but `subint_proc()` raises.
+
+Status
+------
+SCAFFOLDING STUB — `subint_proc()` is **not yet implemented**.
+The real impl lands in Phase B.2 (see issue #379).
+
+'''
+from __future__ import annotations
+import sys
+from typing import (
+    Any,
+    TYPE_CHECKING,
+)
+
+import trio
+from trio import TaskStatus
+
+
+try:
+    from concurrent import interpreters as _interpreters  # type: ignore
+    _has_subints: bool = True
+except ImportError:
+    _interpreters = None  # type: ignore
+    _has_subints: bool = False
+
+
+if TYPE_CHECKING:
+    from tractor.discovery._addr import UnwrappedAddress
+    from tractor.runtime._portal import Portal
+    from tractor.runtime._runtime import Actor
+    from tractor.runtime._supervise import ActorNursery
+
+
+async def subint_proc(
+    name: str,
+    actor_nursery: ActorNursery,
+    subactor: Actor,
+    errors: dict[tuple[str, str], Exception],
+
+    # passed through to actor main
+    bind_addrs: list[UnwrappedAddress],
+    parent_addr: UnwrappedAddress,
+    _runtime_vars: dict[str, Any],  # serialized and sent to _child
+    *,
+    infect_asyncio: bool = False,
+    task_status: TaskStatus[Portal] = trio.TASK_STATUS_IGNORED,
+    proc_kwargs: dict[str, any] = {}
+
+) -> None:
+    '''
+    Create a new sub-actor hosted inside a PEP 734
+    sub-interpreter running in a dedicated OS thread,
+    reusing tractor's existing UDS/TCP IPC handshake
+    for parent<->child channel setup.
+
+    NOT YET IMPLEMENTED — placeholder stub pending the
+    Phase B.2 impl.
+
+    '''
+    if not _has_subints:
+        raise RuntimeError(
+            f'The {"subint"!r} spawn backend requires Python 3.14+ '
+            f'(stdlib `concurrent.interpreters`, PEP 734).\n'
+            f'Current runtime: {sys.version}'
+        )
+
+    raise NotImplementedError(
+        'The `subint` spawn backend scaffolding is in place but '
+        'the spawn-flow itself is not yet implemented.\n'
+        'Tracking: https://github.com/goodboy/tractor/issues/379'
+    )