Expand description
Process-wide registry of live spawned-child PIDs.
Keyed by a monotonic AtomicU64 counter rather than Child::id()
because POSIX recycles PIDs aggressively on long-running runners; a
recycled PID would collide with a previously-deregistered entry.
Stores PIDs (not Child handles): the ScopedChild wrapper owns
the Child outright so it can call wait_with_output / wait
normally, and the signal handler kills by PID via a kill -9 <pid> shell subprocess on Unix (avoids adding libc as a
workspace dep) or OpenProcess + TerminateProcess on Windows. No
ownership transfer, no race between wait and kill.