The PID-reuse guard refused to signal whenever the current reading differed
from the one taken at spawn, including when it was ABSENT. On Linux a reaped
leader has no /proc/<pid>/stat, so every teardown after the leader exited
skipped SIGTERM/SIGKILL while the group it led still held survivors -- the
exact case the process-group teardown exists to reap. Three same-group survivor
tests went red on the coverage lane; they pass on Darwin because the reader
always returns undefined there, leaving the guard inert.
Only a present-and-different reading now blocks the signal. Verified on the
self-hosted Linux box: a reaped leader with live survivors allows the signal, a
pid whose start time differs still blocks it, and a live matching process is
signalled.
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