SIGKILL bounds a sync Popen.wait() that directly follows kill(), so it no
longer needs a timeout. communicate() after kill() and asyncio's
Process.wait() after kill() stay flagged: both read/close the pipes, and a
grandchild holding them blocks forever (measured: > 20 s with a background
`sleep`; the sync wait() returned in 0.00 s). The HX006 fix text now names
the reap pattern.