Since "an old Desktop never reads a dead owner while the POSIX hand-off dies", posix.sh hands the
update marker to a custodian process that outlives the hand-off and logs
"update marker names its custodian pid N (hand-off pid M)". The producer-receipt test still waited
for the marker to name the hand-off script's own pid and failed at the refresh assertion. It now
requires that log line for this hand-off and asserts the refreshed marker names that custodian.
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