The self-test now runs sync_package over the checked-in mise-bin package --
its real metadata and PKGBUILD, the full selection/validation/backstop/
rewrite/read-back path -- with only the two network fetches replaced by
mise-shaped fixtures, asserting the final PKGBUILD holds the quarantine-
cleared version, pkgrel 1, and both architecture checksums.
Review fixes: the release-row builder uses "" fallbacks instead of empty
so a malformed row cannot shift columns past the per-field checks, and
provider discovery now keys on the presence of an upstream declaration
rather than a well-formed one, with sync_package failing loudly on a
declaration it cannot use -- a malformed manifest can no longer silently
drop a package out of scheduled synchronization.