## Summary
Adds a local `just rebuild` recipe to (re)build specific versions of a single package on a given OS/arch, dispatching to a remote buildx builder (the build runs there, not locally).
- `just rebuild <os> <tag> <arch> <package> <version>...` → `docker buildx build --builder <artemis|gaia> --platform linux/<arch> …` (amd64→artemis, arm64→gaia; names + `R_VERSION` env-overridable).
- `docker/build-one.Dockerfile` runs `build-one.R` as a secret-mounted `RUN`, built `--no-cache --output type=cacheonly` (pure side-effect: the S3 upload; no image kept).
- `local/build-one.R` auto-classifies each version via the ABI classifier (risky → per-minor slot `contrib/<x.y>/`, else generic), force-rebuilds + uploads + stores metadata, then refreshes the touched slot's `PACKAGES` index.
## Prerequisites
- buildx builders named `artemis` (amd64) and `gaia` (arm64) registered (`docker buildx create --name artemis ssh://…`).
- Exported secrets: `B2_S3_ACCESS_KEY`, `B2_S3_SECRET_KEY`, `PGPASS` (`GITHUB_PAT` optional).
- bincraft `v4.2.0` tag must exist (the build installs `@v4.2.0` and uses its classifier + per-minor index API).
Reviewed-on: #87
