Simplify stdio client shutdown internals

- Wait for process exit with a plain deadline-polling loop instead of a
  cancel scope: the timeout path no longer throws through the await chain,
  which removes the coverage-resync checkpoint it needed
  (python/cpython#106749) and collapses the double-exit/re-check shape
  into a single while loop
- Extract the reader's shutdown drain (_drain_stdout), the tolerant pipe
  closes (_close_pipe, replacing two pragma-guarded inline blocks), the
  POSIX group-liveness probe (_group_alive), and Job Object handle closing
  (_close_job_handle)
- Add a module docstring telling the shutdown story once and trim comments
  to the constraints they pin

No behavior change intended; the full suite, coverage, and the stdio
stress runs are unchanged.

Author: maxisbey