newestArchiveOn's (time.Time, bool) signature could not express the 'unknown'
its own doc comment promised: a read error and a genuine not-found both returned
(zero,false), so /backup/due emitted a POSITIVE 'no successful backup recorded
yet' with a nil age, and the controller fired its window-gate valve on an
unreadable storage.
Three states now: known / absent / unknown, carried as a STRING enum so the zero
value unambiguously means 'legacy agent' rather than masquerading as an answer.
Fail-safe direction unchanged — unknown is still DUE; only the window-gate bypass
narrows to ABSENT.
A service with NO lister deliberately stays ABSENT: calling it unknown would stop
a genuinely new box on a pre-R-84 build from ever backing up outside its window.
An unparseable timestamp becomes unknown — a backup happened, we cannot date it.
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