Honor `TRACTOR_LOGLEVEL`+`TRACTOR_SPAWN_METHOD` env-vars

Add env-var overrides inside `._root.open_root_actor()` so
devs/test-runs can swap the actor-spawn backend or crank
console verbosity *without* touching application code.

In `._root.open_root_actor()`,
- read `TRACTOR_LOGLEVEL` early, overriding any caller-passed
  `loglevel` and stashing an `env_ll_report` to emit once the
  console log is set up.
- pull the `loglevel` fallback (`or _default_loglevel`) and
  `log.get_console_log()` init *up* so the env-var report
  routes through tractor's own logger.
- read `TRACTOR_SPAWN_METHOD`, overriding any caller-passed
  `start_method` and warn-logging when the env-var clobbers
  an explicit caller value.

Wire the same vars through `tests/devx/conftest.py::spawn`,
- request the `loglevel` fixture, set both `TRACTOR_LOGLEVEL`
  and `TRACTOR_SPAWN_METHOD` in `os.environ` before each
  `pexpect.spawn()` (inherited by the example subproc).
- expand `supported_spawners` to include
  `main_thread_forkserver` and `subint_forkserver` bc
  example scripts no longer need per-script CLI plumbing.
- pop both vars in fixture teardown so a leaked value can't
  re-route a later in-process tractor test's spawn-backend
  or loglevel.

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

tests/devx/conftest.py

@@ -56,6 +56,7 @@ type PexpectSpawner = Callable[
 @pytest.fixture
 def spawn(
     start_method: str,
+    loglevel: str,
     testdir: pytest.Pytester,
     reg_addr: tuple[str, int],
 
@@ -67,11 +68,12 @@ def spawn(
     '''
     supported_spawners: set[str] = {
         'trio',
-        # ?TODO, other spawners that will work?
+        # `examples/debugging/<script>.py` picks up the spawn
-        # - [ ] need to pass `start_method={spawner}` to underlying
+        # backend via the `TRACTOR_SPAWN_METHOD` env-var which
-        #      `examples/debugging/<script>.py` somehow?
+        # is honored inside `tractor._root.open_root_actor()`,
-        # 'main_thread_forkserver',
+        # so no per-script edits are required.
-        # 'subint_forkserver',
+        'main_thread_forkserver',
+        'subint_forkserver',
     }
     if start_method not in supported_spawners:
         pytest.skip(
@@ -94,6 +96,32 @@ def spawn(
         # disable all ANSI color output
         # os.environ['NO_COLOR'] = '1'
 
+    def set_spawn_method():
+        '''
+        Drive the actor-spawn backend inside the spawned
+        `examples/debugging/<script>.py` subproc via env-var
+        (consumed by `tractor._root.open_root_actor()`),
+        without requiring per-script CLI plumbing.
+
+        '''
+        import os
+        os.environ['TRACTOR_SPAWN_METHOD'] = start_method
+
+    def set_loglevel():
+        '''
+        Forward the test-suite parametrized `loglevel` into the
+        spawned `examples/debugging/<script>.py` subproc via
+        env-var (consumed by `tractor._root.open_root_actor()`),
+        so console verbosity can be cranked or silenced from
+        the test harness without per-script edits.
+
+        '''
+        import os
+        if loglevel:
+            os.environ['TRACTOR_LOGLEVEL'] = loglevel
+        else:
+            os.environ.pop('TRACTOR_LOGLEVEL', None)
+
     spawned: PexpectSpawner|None = None
 
     def _spawn(
@@ -103,6 +131,8 @@ def spawn(
     ) -> pty_spawn.spawn:
         nonlocal spawned
         unset_colors()
+        set_spawn_method()
+        set_loglevel()
         spawned = testdir.spawn(
             cmd=mk_cmd(
                 cmd,
@@ -146,6 +176,16 @@ def spawn(
         if ptyproc.isalive():
             ptyproc.kill(signal.SIGKILL)
 
+    # Scope our env-var mutations to this single fixture
+    # invocation — both `TRACTOR_SPAWN_METHOD` and
+    # `TRACTOR_LOGLEVEL` are honored by
+    # `tractor._root.open_root_actor()` so leaking them past
+    # this test could inadvertently re-route a later
+    # in-process tractor test's spawn-backend / loglevel.
+    import os
+    os.environ.pop('TRACTOR_SPAWN_METHOD', None)
+    os.environ.pop('TRACTOR_LOGLEVEL', None)
+
     # TODO? ensure we've cleaned up any UDS-paths?
     # breakpoint()
 

tractor/_root.py

@@ -241,6 +241,7 @@ async def open_root_actor(
             f'_registry_addrs: {registry_addrs!r}\n'
         )
 
+    # debug.mk_pdb().set_trace()
     async with maybe_block_bp(
         debug_mode=debug_mode,
         maybe_enable_greenback=maybe_enable_greenback,
@@ -284,6 +285,75 @@ async def open_root_actor(
             )
             enable_modules.extend(rpc_module_paths)
 
+        # `TRACTOR_LOGLEVEL` env-var wins over any caller-passed
+        # `loglevel` so devs/test-runs can crank (or silence)
+        # console verbosity without touching application code.
+        env_ll_report: str = ''
+        if env_ll := os.environ.get('TRACTOR_LOGLEVEL'):
+            loglevel = env_ll
+            env_ll_report: str = (
+                f'Detected env-var setting,\n'
+                f'TRACTOR_LOGLEVEL={env_ll!r}\n'
+                f'\n'
+                f'Setting console loglevel per,\n'
+                f'loglevel={loglevel!r}\n'
+            )
+            if (
+                loglevel
+                and
+                loglevel.upper() != env_ll.upper()
+            ):
+                env_ll_report += (
+                    f'\n'
+                    f'NOTE env-var OVERRIDES caller-passed,\n'
+                    f'loglevel={loglevel!r}\n'
+                )
+
+        loglevel: str = (
+            loglevel
+            or
+            log._default_loglevel
+        )
+        loglevel: str = loglevel.upper()
+
+        assert loglevel
+        _log = log.get_console_log(
+            level=loglevel,
+            name='tractor',
+            logger=logger,
+        )
+        assert _log
+        if env_ll_report:
+            _log.info(env_ll_report)
+
+        # `TRACTOR_SPAWN_METHOD` env-var wins over any caller-passed
+        # `start_method` so devs/test-runs can swap the actor spawn
+        # backend without touching application code (e.g. driving
+        # the `examples/debugging/<script>.py` suite under each
+        # backend from `tests/devx/conftest.py::spawn`).
+        if env_sm := os.environ.get('TRACTOR_SPAWN_METHOD'):
+            start_method: str = env_sm
+            env_sm_report: str = (
+                f'Detected env-var setting,\n'
+                f'TRACTOR_SPAWN_METHOD={env_sm!r}\n'
+                f'\n'
+                f'Setting spawn backend as,\n'
+                f'start_method={env_sm!r}\n'
+            )
+            if (
+                start_method
+                and
+                start_method != env_sm
+            ):
+                _log.warning(
+                    env_sm_report
+                    +
+                    f'NOTE env-var OVERRIDES caller-passed,\n'
+                    f'`start_method={start_method!r}`\n'
+                )
+            else:
+                _log.info(env_sm_report)
+
         if start_method is not None:
             _spawn.try_set_start_method(start_method)
 
@@ -300,12 +370,6 @@ async def open_root_actor(
             wrap_address(uw_addr)
             for uw_addr in uw_reg_addrs
         ]
-        loglevel: str = (
-            loglevel
-            or
-            log._default_loglevel
-        )
-        loglevel: str = loglevel.upper()
 
         # Debug-mode is currently only supported for backends whose
         # subactor root runtime is trio-native (so `tractor.devx.
@@ -341,13 +405,6 @@ async def open_root_actor(
                 f'{_spawn._spawn_method!r}.'
             )
 
-        assert loglevel
-        _log = log.get_console_log(
-            level=loglevel,
-            name='tractor',
-        )
-        assert _log
-
         # TODO: factor this into `.devx._stackscope`!!
         if (
             debug_mode