Problem

ResourceTracker.__del__ (added in pythongh-88887) calls os.waitpid(pid, 0)
which blocks indefinitely if a process created via os.fork() still
holds the tracker pipe's write end. The tracker never sees EOF, never
exits, and the parent hangs at interpreter shutdown.

Root cause

Three requirements conflict:

- pythongh-88887 wants the parent to reap the tracker to prevent zombies
- pythongh-80849 wants mp.Process(fork) children to reuse the parent's
  tracker via the inherited pipe fd
- pythongh-146313 shows the parent can't block in waitpid() if a child
  holds the fd -- the tracker won't see EOF until all copies close

Fix

Two layers:

Timeout safety-net. _stop_locked() gains a wait_timeout parameter.
When called from __del__, it polls with WNOHANG using exponential
backoff for up to 1 second instead of blocking indefinitely.

At-fork handler. An os.register_at_fork(after_in_child=...) handler
closes the inherited pipe fd in the child unless a preserve flag is
set. popen_fork.Popen._launch() sets the flag before its fork so
mp.Process(fork) children keep the fd and reuse the parent's tracker
(preserving pythongh-80849). Raw os.fork() children close the fd, letting
the parent reap promptly.

Result

  Scenario                                       Before     After
  Raw os.fork(), parent exits while child alive  deadlock   ~30ms reap
  mp.Process(fork), parent joins then exits      ~30ms reap ~30ms reap
  mp.Process(fork), parent exits abnormally      deadlock   1s bounded wait
  No fork (pythongh-88887 scenario)                    ~30ms reap ~30ms reap

The at-fork handler makes the timeout unreachable in all well-behaved
paths. The timeout remains as a safety net for abnormal shutdowns.