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-install-deps.yaml

@@ -36,6 +36,13 @@ variables:
       - 4.5.3
       - 4.4.3
     default: 4.5.3
+  SEED_FROM:
+    # Only for bootstrapping a new slot. Copies the pure-R binaries of these
+    # slots (same OS family and arch, same primary R minor; see
+    # local/seed-pure-r.R) before the build list is computed, so build-all only
+    # compiles what actually needs compiling. Space-separated, first wins.
+    description: 'Existing slots to copy pure-R binaries from, e.g. "alpine324 alpine323". Empty: no seeding.'
+    default: ''
 
 when:
   - event: manual
@@ -88,6 +95,7 @@ steps:
       - /opt/R/$R_VERSION/bin/Rscript local/install-bincraft.R
       - UVR_R_BIN=/opt/R/$R_VERSION/bin/R local/uvr-install.sh RPostgres s3fs data.table future jsonlite purrr
       - /opt/R/$R_VERSION/bin/R -q -e 'packageVersion("bincraft")'
+      - if [ -n "${SEED_FROM}" ]; then /opt/R/$R_VERSION/bin/Rscript local/seed-pure-r.R ${SEED_FROM}; fi
       - /opt/R/$R_VERSION/bin/R -q -e "source('local/packages-to-build.R'); saveRDS(pkgs, '/mnt/cache/packages/pkgs_to_build.rds'); saveRDS(pkgs[r_minor_sensitive == TRUE], '/mnt/cache/packages/r_minor_sensitive_pkgs.rds'); sprintf('Precomputed %s package versions (%s r-minor-sensitive)', nrow(pkgs), nrow(pkgs[r_minor_sensitive == TRUE]))"
     backend_options:
       docker: