Extract `_actor_child_main()` as shared child entry

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)

tractor/_child.py

@@ -15,16 +15,23 @@
 # along with this program.  If not, see <https://www.gnu.org/licenses/>.
 
 """
-This is the "bootloader" for actors started using the native trio backend.
+The "bootloader" for sub-actors spawned via the native `trio`
+backend (the default `python -m tractor._child` CLI entry) and
+the in-process `subint` backend (`tractor.spawn._subint`).
 
 """
+from __future__ import annotations
 import argparse
-
 from ast import literal_eval
+from typing import TYPE_CHECKING
 
 from .runtime._runtime import Actor
 from .spawn._entry import _trio_main
 
+if TYPE_CHECKING:
+    from .discovery._addr import UnwrappedAddress
+    from .spawn._spawn import SpawnMethodKey
+
 
 def parse_uid(arg):
     name, uuid = literal_eval(arg)  # ensure 2 elements
@@ -39,6 +46,36 @@ def parse_ipaddr(arg):
         return arg
 
 
+def _actor_child_main(
+    uid: tuple[str, str],
+    loglevel: str | None,
+    parent_addr: UnwrappedAddress | None,
+    infect_asyncio: bool,
+    spawn_method: SpawnMethodKey = 'trio',
+
+) -> None:
+    '''
+    Construct the child `Actor` and dispatch to `_trio_main()`.
+
+    Shared entry shape used by both the `python -m tractor._child`
+    CLI (trio/mp subproc backends) and the `subint` backend, which
+    invokes this from inside a fresh `concurrent.interpreters`
+    sub-interpreter via `Interpreter.call()`.
+
+    '''
+    subactor = Actor(
+        name=uid[0],
+        uuid=uid[1],
+        loglevel=loglevel,
+        spawn_method=spawn_method,
+    )
+    _trio_main(
+        subactor,
+        parent_addr=parent_addr,
+        infect_asyncio=infect_asyncio,
+    )
+
+
 if __name__ == "__main__":
     __tracebackhide__: bool = True
 
@@ -49,15 +86,10 @@ if __name__ == "__main__":
     parser.add_argument("--asyncio", action='store_true')
     args = parser.parse_args()
 
-    subactor = Actor(
-        name=args.uid[0],
-        uuid=args.uid[1],
+    _actor_child_main(
+        uid=args.uid,
         loglevel=args.loglevel,
-        spawn_method="trio"
-    )
-
-    _trio_main(
-        subactor,
         parent_addr=args.parent_addr,
         infect_asyncio=args.asyncio,
+        spawn_method='trio',
     )

tractor/runtime/_runtime.py

@@ -870,7 +870,7 @@ class Actor:
 
             accept_addrs: list[UnwrappedAddress]|None = None
 
-            if self._spawn_method == "trio":
+            if self._spawn_method in ("trio", "subint"):
 
                 # Receive post-spawn runtime state from our parent.
                 spawnspec: msgtypes.SpawnSpec = await chan.recv()