Enable `debug_mode` for `subint_forkserver`

The `subint_forkserver` backend's child runtime is trio-native (uses
`_trio_main` + receives `SpawnSpec` over IPC just like `trio`/`subint`),
so `tractor.devx.debug._tty_lock` works in those subactors. Wire the
runtime gates that historically hard-coded `_spawn_method == 'trio'` to
recognize this third backend.

Deats,
- new `_DEBUG_COMPATIBLE_BACKENDS` module-const in `tractor._root`
  listing the spawn backends whose subactor runtime is trio-native
  (`'trio'`, `'subint_forkserver'`). Both the enable-site
  (`_runtime_vars['_debug_mode'] = True`) and the cleanup-site reset
  key.
  off the same tuple — keep them in lockstep when adding backends
- `open_root_actor`'s `RuntimeError` for unsupported backends now
  reports the full compatible-set + the rejected method instead of the
  stale "only `trio`" msg.
- `runtime._runtime.Actor._from_parent`'s SpawnSpec-recv gate adds
  `'subint_forkserver'` to the existing `('trio', 'subint')` tuple
  — fork child-side runtime receives the same SpawnSpec IPC handshake as
  the others.
- `subint_forkserver_proc` child-target now passes
  `spawn_method='subint_forkserver'` (was hard-coded `'trio'`) so
  `Actor.pformat()` / log lines reflect the actual parent-side spawn
  mechanism rather than masquerading as plain `trio`.

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

tractor/_root.py

@@ -69,6 +69,20 @@ from ._exceptions import (
 logger = log.get_logger('tractor')
 
 
+# Spawn backends under which `debug_mode=True` is supported.
+# Requirement: the spawned subactor's root runtime must be
+# trio-native so `tractor.devx.debug._tty_lock` works. Matches
+# both the enable-site in `open_root_actor` and the cleanup-
+# site reset of `_runtime_vars['_debug_mode']` — keep them in
+# lockstep when adding backends.
+_DEBUG_COMPATIBLE_BACKENDS: tuple[str, ...] = (
+    'trio',
+    # forkserver children run `_trio_main` in their own OS
+    # process — same child-side runtime shape as `trio_proc`.
+    'subint_forkserver',
+)
+
+
 # TODO: stick this in a `@acm` defined in `devx.debug`?
 # -[ ] also maybe consider making this a `wrapt`-deco to
 #     save an indent level?
@@ -293,10 +307,14 @@ async def open_root_actor(
         )
         loglevel: str = loglevel.upper()
 
+        # Debug-mode is currently only supported for backends whose
+        # subactor root runtime is trio-native (so `tractor.devx.
+        # debug._tty_lock` works). See `_DEBUG_COMPATIBLE_BACKENDS`
+        # module-const for the list.
         if (
             debug_mode
             and
-            _spawn._spawn_method == 'trio'
+            _spawn._spawn_method in _DEBUG_COMPATIBLE_BACKENDS
         ):
             _state._runtime_vars['_debug_mode'] = True
 
@@ -318,7 +336,9 @@ async def open_root_actor(
 
         elif debug_mode:
             raise RuntimeError(
-                "Debug mode is only supported for the `trio` backend!"
+                f'Debug mode currently supported only for '
+                f'{_DEBUG_COMPATIBLE_BACKENDS!r} spawn backends, not '
+                f'{_spawn._spawn_method!r}.'
             )
 
         assert loglevel
@@ -619,7 +639,7 @@ async def open_root_actor(
             if (
                 debug_mode
                 and
-                _spawn._spawn_method == 'trio'
+                _spawn._spawn_method in _DEBUG_COMPATIBLE_BACKENDS
             ):
                 _state._runtime_vars['_debug_mode'] = False
 

tractor/runtime/_runtime.py

@@ -870,7 +870,14 @@ class Actor:
 
             accept_addrs: list[UnwrappedAddress]|None = None
 
-            if self._spawn_method in ("trio", "subint"):
+            if self._spawn_method in (
+                'trio',
+                'subint',
+                # `subint_forkserver` parent-side sends a
+                # `SpawnSpec` over IPC just like the other two
+                # — fork child-side runtime is trio-native.
+                'subint_forkserver',
+            ):
 
                 # Receive post-spawn runtime state from our parent.
                 spawnspec: msgtypes.SpawnSpec = await chan.recv()