Exhausting the 12h budget called writeAbsent unconditionally, so an
entity whose art was already resolved and serving lost it to a long
upstream outage: the hash went empty, clients fell back to the
placeholder, and the now-unreferenced bytes were freed by the next
prune even though nothing about the image had changed.
Exhaustion means the source stayed unreachable, not that the cover
disappeared, so absent is now recorded only when there is nothing to
keep.
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