Drop `debug_mode` gate on stackscope SIGUSR1

SIGUSR1 task-tree dumps via `stackscope` should work in
plain (non-pdb) runs too — esp. in infected-`asyncio`
processes where the kernel-default SIGUSR1 disposition is
`Term` (proc dies on `kill -USR1` w/o an installed
handler). Ungate the install path from `_debug_mode` in
both root and sub-actor init; the `use_stackscope` rt-var
+ `TRACTOR_ENABLE_STACKSCOPE` env-var checks remain as
the actual opt-in (e.g. via `--enable-stackscope`).

Deats,
- `_root.open_root_actor`: drop the `debug_mode and ...`
  conjunction around the `enable_stack_on_sig()` call;
  now gated only on the `enable_stack_on_sig` arg itself.
- `_runtime.Actor` sub-actor init: lift the
  `use_stackscope`/`TRACTOR_ENABLE_STACKSCOPE` branch out
  of the `if rvs['_debug_mode']:` block to peer-level.
  The `use_greenback` branch stays inside `_debug_mode`
  (pdb-specific).
- Refresh inline comments on both sites to call out the
  infected-`asyncio` "default SIGUSR1 = terminate proc"
  rationale.

(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 3d9c75b6ed)

tractor/_root.py

@@ -435,11 +435,12 @@ async def open_root_actor(
             )
 
         # TODO: factor this into `.devx._stackscope`!!
-        if (
+        #
-            debug_mode
+        # NOTE, intentionally NOT gated on `debug_mode` so SIGUSR1
-            and
+        # task-tree dumps work in plain (non-pdb) runs too — esp.
-            enable_stack_on_sig
+        # in infected-`asyncio` root processes where the default
-        ):
+        # SIGUSR1 action would otherwise terminate the proc.
+        if enable_stack_on_sig:
             from .devx._stackscope import enable_stack_on_sig
             enable_stack_on_sig()
 

tractor/runtime/_runtime.py

@@ -931,13 +931,18 @@ class Actor:
                 rvs: dict[str, Any] = spawnspec._runtime_vars
 
                 # `stackscope` SIGUSR1 handler: install when EITHER
-                # `_debug_mode=True` (full multi-actor pdb support
+                # `use_stackscope` is set in rt-vars OR the
-                # path) OR the `TRACTOR_ENABLE_STACKSCOPE` env var
+                # `TRACTOR_ENABLE_STACKSCOPE` env var is set (lighter
-                # is set (lighter test-time hang-debug path; see
+                # test-time hang-debug path; see
                 # `tractor._testing.pytest`'s `--enable-stackscope`
                 # CLI flag — env var propagates via fork-inherited
                 # environ).
-                if rvs['_debug_mode']:
+                #
+                # NOTE, intentionally NOT gated on `_debug_mode` so
+                # SIGUSR1 task-tree dumps work in plain (non-pdb)
+                # runs too — esp. in infected-`asyncio` sub-actors
+                # where the default SIGUSR1 action would otherwise
+                # terminate the proc.
                 if (
                     rvs.get('use_stackscope')
                     or
@@ -959,6 +964,7 @@ class Actor:
                             'debug mode / `--enable-stackscope`!'
                         )
 
+                if rvs['_debug_mode']:
                     if rvs.get('use_greenback', False):
                         from ..devx import maybe_init_greenback
                         maybe_mod: ModuleType|None = await maybe_init_greenback()