MacProcessInspector answered the descendant tree and every member's
liveness with its own `/bin/ps` fork, so one readiness poll cost N+1
full table reads for N tracked descendants. With execFileSync on that
path and a 50 ms poll interval, any command spawning two or more
children saturated the host event loop until it exited.
ProcessInspector.snapshot() now returns one ProcessSnapshot that
answers tree, session, and alive from a single observation, and
signalProcess takes the caller's observation so its PID-reuse fence
does not re-read the table per member.
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