Clarify eg-wrap contract + tighten `stdout` typing

Doc/type accuracy in `supervise_run_process()`:

- the docstring claimed the rc!=0 `CalledProcessError` had
  "no nursery-eg-wrapped CPE to catch/`collapse_eg`", but it
  IS raised as a task into the parent `tn`, so under
  `trio>=0.33` it surfaces `ExceptionGroup`-wrapped like any
  `tn.start()`-task raise. Say so + point at `except*` /
  `collapse_eg()` (matching the inline rc-check comment).
- widen `stdout: int` to `int|None` — the param accepts an
  explicit `None` (inherit) per its own docstring.
- note that stdout is NOT captured, so the CPE note carries
  only stderr (a cmd logging errors to stdout should
  `relay_stdout=True`).

Review: PR #465 (copilot-pull-request-reviewer)
https://github.com/goodboy/tractor/pull/465#pullrequestreview-4548191641

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

tractor/trionics/_subproc.py

@@ -160,7 +160,7 @@ async def supervise_run_process(
     # non-relay `stdout` override; defaults (via `_UNSET`) to
     # `DEVNULL` so we NEVER inherit (+ thus can't clobber) the
     # parent controlling-tty.
-    stdout: int = _UNSET,
+    stdout: int|None = _UNSET,
 
     task_status: trio.TaskStatus[
         trio.Process
@@ -178,11 +178,14 @@ async def supervise_run_process(
 
     - surfaces a rc!=0 `subprocess.CalledProcessError`
       DETERMINISTICALLY: we pass `check=False` to `trio` and
-      do our OWN post-drain rc-check, (re)building + raising a
+      do our OWN post-drain rc-check, (re)building + raising the
-      BARE CPE (with a `.stderr` note) from this coro's body
+      CPE (with a `.stderr` note) from this coro's body AFTER the
-      AFTER the child exits — so there's no nursery-eg-wrapped
+      child exits — so it's never wrapped by `trio.run_process`'s
-      CPE to catch/`collapse_eg`, and the relay reader is never
+      INTERNAL nursery nor race-cancelled mid-drain. It IS still
-      race-cancelled mid-drain.
+      raised as a task into the *parent* `tn`, so — like any
+      `tn.start()`-task raise — it surfaces `ExceptionGroup`-
+      wrapped under `trio>=0.33`; unwrap via `except*` or
+      `trionics.collapse_eg()`.
 
     - ALWAYS isolates the parent controlling-tty
       (`stdin=DEVNULL`, and `stdout=DEVNULL` unless
@@ -339,6 +342,10 @@ async def supervise_run_process(
             if stderr_accum is not None
             else b''
         )
+        # NOTE: stdout is NOT captured (DEVNULL unless overridden)
+        # so the CPE carries only (tail-bounded) stderr; a cmd that
+        # logs failures to *stdout* should `relay_stdout=True` to
+        # surface them in the note.
         cpe = subprocess.CalledProcessError(
             returncode=trio_proc.returncode,
             cmd=trio_proc.args,