cran-rpkgs-test was added as a second pull zone on the same B2 origin but
never added to the purge list, so it kept serving pre-reindex indexes
behind the same ~370 day cache_expiration_time as production.
That is not cosmetic: a verification run against staging measures whatever
the edge still holds. amd64/alpine323 reported 161 regressions there while
production, freshly purged, reported zero for the same objects. The
staging zone exists to be measured, so it has to be purged like any other.
## Motivation
`weekly-rebuild-reindex` exists to run after `weekly-rebuild-missing`: it declares `depends_on: weekly-rebuild-missing` and is gated on that workflow's `weekly_rebuild_missing` variable. Triggering it manually therefore also starts hours of package rebuilds.
That is the wrong tool when only the index needs regenerating. After rpkgs/bincraft#113 (v5.1.5), which changes how `union_index_records()` decides what a per-minor index steers to, every object in the bucket is already correct and only `PACKAGES*` is stale. Rebuilding to fix an index is pure waste, and the natural cron would take a full cycle to reach every slot.
## Change
Adds `.crow/reindex.yaml`: the index half on its own, manual only, no dependency on a rebuild.
It reuses the same matrix and the same steps as `weekly-rebuild-reindex` — install the latest bincraft release, republish the generic index, loop the installed R versions republishing each per-minor index, purge the edge. No package is built.
Gated on a new `reindex` variable so it cannot be started by the rebuild gate, defaulting to `none` so a manual pipeline creation (which instantiates every file in `.crow/`) matches no matrix row.
```sh
crow pipeline create devxy/build-cran-binaries --var reindex=all
crow pipeline create devxy/build-cran-binaries --var reindex=ubuntu-2404-amd64
```
## Verification
- `crow lint .crow/` passes.
- Gate is manual-only and evaluates `reindex`, with no `depends_on` and no `runs_on` carried over from the rebuild coupling.
Reviewed-on: #177
