bin/sync-upstream self-test swaps the two network fetches in
helpers/upstream-github.sh for fixture readers and runs the production code
paths: fallback past a quarantined release, draft/prerelease filtering, the
deliberate bypass, unchanged-version and all-quarantined no-update paths,
unusable tags/timestamps and missing checksums failing the sync, {tag} and
{pkgver} asset templates with ./ and * manifest prefixes across both
architectures, the min_release_age backstop verdicts (now a testable
release_age_status function), the duration parser, and manifest validation.
Also fixes from the review: the duration parser forces base-10 arithmetic
(leading zeros no longer parse as octal) and bounds values to nine digits so
no suffix can overflow; jq // treating false as absent can no longer let
"min_release_age": false or "upstream": false slip through as unset; the
release feed page grew to the API maximum of 100 with the bounded search
documented; and the README package-metadata section documents the upstream
block, min_release_age, the bypass, and provider-versus-hook exclusivity.