Before update_cron was wired in, the plugin wrote directly to
/etc/cron.d/varaverk. That file persisted after the migration and
caused duplicate job firing (two watchdog instances at :00/:15/:30/:45).
The second instance hit the lock, exited with warn, and overwrote the
stat file — killing the running indicator while the first was still live.
vv_cron_rebuild() now unlinks the legacy file after every update_cron
call. Since rebuild_cron (boot event) calls vv_cron_rebuild(), the
cleanup runs on every reboot and every schedule save.