Add `use_stackscope` runtime var for subactor init

Track `stackscope` enablement in `RuntimeVars` so
the flag propagates to subactors via the standard
rtvar IPC path instead of relying solely on the
`TRACTOR_ENABLE_STACKSCOPE` env var.

Deats,
- add `use_stackscope: bool` to `RuntimeVars`
  struct + defaults dict
- `enable_stack_on_sig()` sets the rtvar on
  successful `stackscope` import, asserts unset
  on `ImportError`
- nest stackscope init under `_debug_mode` gate
  in `Actor.async_main`, check rtvar alongside
  env var
- defer `maybe_init_greenback` import to its own
  `use_greenback` branch

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

tractor/devx/_stackscope.py

@@ -381,11 +381,13 @@ def enable_stack_on_sig(
                 message=r"coroutine method '(asend|athrow)' .* was never awaited",
             )
             import stackscope
+            _state._runtime_vars['use_stackscope'] = True
     except ImportError:
         log.warning(
             'The `stackscope` lib is not installed!\n'
             '`Ignoring enable_stack_on_sig() call!\n'
         )
+        assert not _state._runtime_vars['use_stackscope']
         return None
 
     # Capture the trio token if we're inside `trio.run`

tractor/runtime/_runtime.py

@@ -940,16 +940,13 @@ class Actor:
                 # `tractor._testing.pytest`'s `--enable-stackscope`
                 # CLI flag — env var propagates via fork-inherited
                 # environ).
-                import os
+                if rvs['_debug_mode']:
                     if (
-                    rvs['_debug_mode']
+                        rvs.get('use_stackscope')
                         or
                         os.environ.get('TRACTOR_ENABLE_STACKSCOPE')
                     ):
-                    from ..devx import (
+                        from ..devx import enable_stack_on_sig
-                        enable_stack_on_sig,
-                        maybe_init_greenback,
-                    )
                         try:
                             # TODO: maybe return some status msgs upward
                             # to that we can emit them in `con_status`
@@ -966,6 +963,7 @@ class Actor:
                             )
 
                     if rvs.get('use_greenback', False):
+                        from ..devx import maybe_init_greenback
                         maybe_mod: ModuleType|None = await maybe_init_greenback()
                         if maybe_mod:
                             log.devx(

tractor/runtime/_state.py

@@ -93,6 +93,7 @@ class RuntimeVars(Struct):
     repl_fixture: bool|Callable = False  # |AbstractContextManager[bool]
     # for `tractor.pause_from_sync()` & `breakpoint()` support
     use_greenback: bool = False
+    use_stackscope: bool = False
 
     # infected-`asyncio`-mode: `trio` running as guest.
     _is_infected_aio: bool = False
@@ -139,8 +140,9 @@ _RUNTIME_VARS_DEFAULTS: dict[str, Any] = {
     # `debug_mode: bool` settings
     '_debug_mode': False,  # bool
     'repl_fixture': False,  # |AbstractContextManager[bool]
-    # for `tractor.pause_from_sync()` & `breakpoint()` support
+    
-    'use_greenback': False,
+    'use_greenback': False,  # `.pause_from_sync()`/`breakpoint()`
+    'use_stackscope': False,  # trio-task-stack dumps on SIGUSR1
 
     # infected-`asyncio`-mode: `trio` running as guest.
     '_is_infected_aio': False,