perf: seed pure-R binaries into a new slot, interleave shards, and stop builds before the timeout

- 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.

.crow/build-all-versions.yaml

@@ -136,11 +136,18 @@ steps:
       R_LIBS_USER: /mnt/cache/R-pkgs
       CCACHE_DIR: /mnt/cache/ccache
       NCPUS: 2
+      # Whole hours the build passes below may use, measured from the step's
+      # start. The repo timeout is 50h; the remainder is left for archiving the
+      # missed packages, so a long run ends cleanly instead of being killed.
+      BUILD_BUDGET_HOURS: 44
     volumes:
       - ${ARCH}-binaries-r-dep-cache-${OS}-${OS_VERSION//./}:/mnt/cache
     commands:
       - git clone -q https://pat-s:$$REPO_RO_TOKEN@git.devxy.io/devxy/build-cran-binaries.git .
       - . local/compiler-cache.sh
+      # One deadline shared by every build pass in this step, read by
+      # local/build-all.R and local/build-newer-r.R before each package.
+      - export BUILD_DEADLINE=$(( $(date +%s) + BUILD_BUDGET_HOURS * 3600 ))
       - mkdir -p /mnt/cache/uvr/cache /mnt/cache/uvr/packages /mnt/cache/R-pkgs /mnt/cache/ccache /mnt/cache/packages
       # The primary pass must not rely on build-all-versions-install-deps having
       # run on *this* agent: depends_on only orders the steps, but the cache