Make SIGUSR1 `stackscope` dumps actually work

Two fixes to the hang-debug SIGUSR1 task-tree dump path,
surfaced by `/code-review high` on #462,

- re-add `_debug_mode` to the sub-actor handler-install gate
  in `_runtime.py`. Dropping it (rel. `3a386ba5`/`3d9c75b6`
  "Drop debug_mode gate", from the `custom_log_levels_api`
  follow-up) was meant to *also* enable non-pdb runs, but
  nothing sets `use_stackscope` from `debug_mode`, so
  debug-mode subs were left with NO handler — and the default
  SIGUSR1 disposition then *kills* them. Now additive:
  `_debug_mode OR use_stackscope OR env`.
- pass `write_file=True` at both `dump_task_tree()` SIGUSR1
  call sites so the advertised `/tmp/tractor-stackscope-<pid>`
  `.log` tee is actually written (was dead under
  `--capture=fd`). Matches `1b1ef10a` "Re-enable writing
  `stackscope` to file by default"; param from `0df90500`.

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

tractor/devx/_stackscope.py

@@ -248,7 +248,7 @@ def _dump_then_relay(
 
     '''
     try:
-        dump_task_tree()
+        dump_task_tree(write_file=True)
     except BaseException:
         log.exception(
             '`dump_task_tree()` raised (scheduled via '
@@ -314,7 +314,7 @@ def dump_tree_on_sig(
                 return
 
             else:
-                dump_task_tree()
+                dump_task_tree(write_file=True)
 
         except RuntimeError:
             log.exception(

tractor/runtime/_runtime.py

@@ -930,20 +930,23 @@ class Actor:
                 # TODO! -[ ] another `Struct` for rtvs..
                 rvs: dict[str, Any] = spawnspec._runtime_vars
 
-                # `stackscope` SIGUSR1 handler: install when EITHER
-                # `use_stackscope` is set in rt-vars OR the
-                # `TRACTOR_ENABLE_STACKSCOPE` env var is set (lighter
-                # test-time hang-debug path; see
-                # `tractor._testing.pytest`'s `--enable-stackscope`
+                # `stackscope` SIGUSR1 handler: install when ANY of
+                # `_debug_mode` / `use_stackscope` rt-vars OR the
+                # `TRACTOR_ENABLE_STACKSCOPE` env var is set (the
+                # latter being a lighter test-time hang-debug path;
+                # see `tractor._testing.pytest`'s `--enable-stackscope`
                 # CLI flag — env var propagates via fork-inherited
                 # environ).
                 #
-                # NOTE, intentionally NOT gated on `_debug_mode` so
+                # NOTE, NOT *exclusively* 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.
+                # runs too — but we DO still install under
+                # `_debug_mode` since otherwise the default SIGUSR1
+                # action would terminate the proc, esp. nasty in
+                # infected-`asyncio` sub-actors mid-REPL.
                 if (
+                    rvs.get('_debug_mode')
+                    or
                     rvs.get('use_stackscope')
                     or
                     os.environ.get('TRACTOR_ENABLE_STACKSCOPE')