Restore fn-arg `_runtime_vars` in `trio_proc` teardown

During the Phase A extraction of `trio_proc()` out of
`spawn._spawn` into its own submod, the
`debug.maybe_wait_for_debugger(child_in_debug=...)` call site in
the hard-reap `finally` got refactored from the original
`_runtime_vars.get('_debug_mode', ...)` (the fn parameter — the
dict that was constructed by the *parent* for the *child*'s
`SpawnSpec`) to `get_runtime_vars().get(...)` (a global getter that
returns the *parent's* live `_state`). Those are semantically
different — the first asks "is the child we just spawned in debug
mode?", the second asks "are *we* in debug mode?". Under
mixed-debug-mode trees the swap can incorrectly skip (or
unnecessarily delay) the debugger-lock wait during teardown.

Revert to the fn-parameter lookup and add an inline `NOTE` comment
calling out the distinction so it's harder to regress again.

Deats,
- `spawn/_trio.py`: `child_in_debug=get_runtime_vars().get(...)` →
  `child_in_debug=_runtime_vars.get(...)` at the
  `debug.maybe_wait_for_debugger(...)` call in the hard-reap block;
  add 4-line `NOTE` explaining the parent-vs-child distinction.
- `spawn/__init__.py`: drop trailing whitespace after the
  `'mp_forkserver'` docstring bullet.
- `ai/prompt-io/prompts/subints_spawner.md`: drop duplicated `with`
  in `"as with with subprocs"` prose (copilot grammar catch).

Review: PR #444 (Copilot)
https://github.com/goodboy/tractor/pull/444#pullrequestreview-4165928469

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

ai/prompt-io/prompts/subints_spawner.md

@@ -24,7 +24,7 @@ Part of this work should include,
     is happening under the hood with how cpython implements subints.
 
   * default configuration should encourage state isolation as with
-    with subprocs, but explicit public escape hatches to enable rigorously
+    subprocs, but explicit public escape hatches to enable rigorously
     managed shm channels for high performance apps.
 
 - all tests should be (able to be) parameterized to use the new

tractor/spawn/_trio.py

@@ -246,7 +246,11 @@ async def trio_proc(
                             await proc.wait()
 
                 await debug.maybe_wait_for_debugger(
-                    child_in_debug=get_runtime_vars().get(
+                    # NOTE: use the child's `_runtime_vars`
+                    # (the fn-arg dict shipped via `SpawnSpec`)
+                    # — NOT `get_runtime_vars()` which returns
+                    # the *parent's* live runtime state.
+                    child_in_debug=_runtime_vars.get(
                         '_debug_mode', False
                     ),
                     header_msg=(