Split every OS/arch row of weekly-rebuild-missing into three shards and move
the build loop out of the inline R one-liner into local/rebuild-missing.R.
A shard re-derives its outstanding set from the bucket on every start: an
object whose ETag equals CRAN's published MD5sum is still a source fallback and
needs building. That is bincraft's check_s3_root_package() evaluated in bulk,
so a restart resumes rather than replaying thousands of per-package HEADs, and
it stays correct when a sibling shard or a process-updates run finishes work in
the meantime. No progress file, no volume, no database cursor.
Give each shard a 20h wall-clock budget so it exits cleanly instead of having
to be killed. A kill matches neither success nor failure, which is how pipeline
10910 skipped its CDN purge and left ~4600 rebuilt binaries behind stale edge
copies.
Move re-indexing and the purge into weekly-rebuild-reindex.yaml, which depends
on the rebuild and runs on success or failure. Three shards writing one slot's
PACKAGES concurrently would race: update_PACKAGES lists the live bucket, so an
early lister that uploads last publishes an index missing its siblings' work.
