perf: seed pure-R binaries into a new slot, interleave shards, and stop builds before the timeout (#242)

Speeds up the full rebuild for a new slot, e.g. the upcoming Alpine version.

## Changes

- **Seed pure-R binaries** (`local/seed-pure-r.R`, `local/seed-helpers.R`), opt-in via the new `SEED_FROM` variable on `build-all-versions-install-deps`.
  About 74% of the ~84k candidate versions have no compiled code and cost ~20s each to build; their R objects are identical across slots.
  Before the build list is computed, each donor candidate is downloaded and verified, then copied server-side:
  - donor must be the same OS family and arch;
  - DESCRIPTION has `Built` (not a CRAN source fallback) with an empty platform and no `libs/`;
  - built by the target's primary R minor;
  - bincraft's `tarball_abi_verdict()` calls it portable across every installed minor (unknown means no seed).

  Each copy gets the `single_builds` and `abi_flat_safety` rows an upload writes, so build-all skips it and the per-minor indexes keep it.
  Re-runs skip existing objects and rewrite rows idempotently.
- **Interleaved shards** in `local/build-all.R` via `shard_slice()`; contiguous alphabetical quarters ranged from 0.1h to 28h.
- **Shared build deadline**: `build-all-versions` sets `BUILD_DEADLINE` from `BUILD_BUDGET_HOURS` (44h, repo timeout 50h); `build-all.R` and `build-newer-r.R` stop cleanly when it passes, so archiving still runs.
- `codename_platform()` replaces the hard-coded codename-to-platform list.

## Testing

- New unit tests for the seed verdict, candidate filter, platform names and deadline; all `local/tests` pass.
- Verdicts checked on real amd64/alpine324 tarballs: R6 2.6.1/2.5.1 and jsonstat seeded, cli refused as compiled.
- `crow lint .crow/` passes.
- The seed has not run against B2/Postgres yet; the first bootstrap run is its end-to-end test.

## Usage

Trigger `build-all-versions` with `target_arch`, `OS`, `OS_VERSION` and `SEED_FROM="alpine324 alpine323"`.
The new Alpine version still needs adding to the `OS_VERSION` options once its image exists.

Reviewed-on: #242

.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:

.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