An object being present in the slot does not mean a binary was built: when a
build fails, bincraft publishes the CRAN source tarball in its place, and the
bucket listing this used could not tell the two apart. That is how
amd64/alpine324 came to hold 13547 CRAN sources that this audit never reported
and weekly-rebuild-missing therefore never retried.
- read the slot's own PACKAGES index instead of listing the bucket, and count
only records carrying a Built stamp, which bincraft writes only for what it
actually built
- report how many records the index holds and how many are served as source
- drop the now-unused s3fs dependency, so the audit needs no B2 credentials
A slot last indexed by a bincraft that predates the matching fix stamps Built
on every record, so this reports exactly what it used to until re-indexed.