Unset debug-mode on root actor exit

Discovered this bug while testing `modden`'s daemon under various
cancelled-while-booting race conditions where sequential tests would
fail a lingering `assert 0` inside `.to_asyncio.run_as_asyncio_guest()`
to (oddly) catch redundant greenback-re-inits..

XD

Needs a test likely ;P

tractor/_root.py

@@ -276,7 +276,8 @@ async def open_root_actor(
 
         if (
             debug_mode
-            and _spawn._spawn_method == 'trio'
+            and
+            _spawn._spawn_method == 'trio'
         ):
             _state._runtime_vars['_debug_mode'] = True
 
@@ -512,8 +513,17 @@ async def open_root_actor(
                     )
                     await actor.cancel(None)  # self cancel
         finally:
+            # revert all process-global runtime state
+            if (
+                debug_mode
+                and
+                _spawn._spawn_method == 'trio'
+            ):
+                _state._runtime_vars['_debug_mode'] = False
+
             _state._current_actor = None
             _state._last_actor_terminated = actor
+
             logger.runtime(
                 f'Root actor terminated\n'
                 f')>\n'