- Add local/seed-pure-r.R, opt-in via SEED_FROM on install-deps: copy pure-R
binaries server-side from a slot of the same OS family and arch, after
verifying each is a binary, has no compiled code, was built by the primary
R minor, and passes bincraft's cross-minor ABI check; write the
single_builds and abi_flat_safety rows an upload would.
- Interleave build-all shards with shard_slice() instead of contiguous
alphabetical blocks, which ranged from 0.1h to 28h per shard.
- Share one BUILD_DEADLINE (BUILD_BUDGET_HOURS, default 44h) across every
build pass of a build-all-versions step so long runs stop cleanly and
still archive and index what they built.
- Derive the single_builds platform with codename_platform() instead of a
hard-coded list.
