validate_listen checked only the port, so FELIS_NANO_LISTEN=::1:8081
passed. Go refuses that form ("too many colons in address") and needs
[::1]:8081, so the unit crash-looped on every start. A host part that
contains a colon must now be in brackets.
With that, the bare ::1 pattern in nano_listen_is_loopback can no
longer match an address that gets this far, so it goes. [::1] stays.
The harness adds ::1:8081 to the refused addresses, and [::]:8081 and
:8081, both of which Go binds, to the accepted ones.
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