OpenWrt keeps its package index in RAM. After a reboot `apk version` only
warns that it cannot open its cached index and exits 0, and `opkg
list-upgradable` prints nothing: both read as "no updates". netOrk keeps
"pending since" per package (MVP 5), so the reader now raises instead:
- apk: on apk's own warning ("opening from cache ... No such file"), or a
failed exit status.
- opkg: when /var/opkg-lists is empty.
- refresh_available_updates(): apk update / opkg update, success from the
exit status.