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

local/tests/test-seed-helpers.R

@@ -0,0 +1,126 @@
+source(file.path("..", "seed-helpers.R"))
+
+# A minimal tarball laid out like R CMD INSTALL --build output.
+make_tarball <- function(
+  package = "R6",
+  version = "2.6.1",
+  built = "R 4.5.3; ; 2026-09-01 10:00:00 UTC; unix",
+  libs = FALSE
+) {
+  root <- tempfile("pkg_")
+  dir.create(file.path(root, package, "R"), recursive = TRUE)
+  fields <- c(Package = package, Version = version)
+  if (!is.na(built)) {
+    fields <- c(fields, Built = built)
+  }
+  write.dcf(t(fields), file.path(root, package, "DESCRIPTION"))
+  writeLines("", file.path(root, package, "R", package))
+  if (libs) {
+    dir.create(file.path(root, package, "libs"))
+    writeLines("", file.path(root, package, "libs", paste0(package, ".so")))
+  }
+  tarball <- tempfile(fileext = ".tar.gz")
+  old <- setwd(root)
+  on.exit(setwd(old), add = TRUE)
+  utils::tar(tarball, package, compression = "gzip", tar = "internal")
+  tarball
+}
+
+portable <- function(t) {
+  list(inspected = TRUE, sensitive = FALSE, unsupported = character())
+}
+
+test_that("seed_verdict accepts a pure-R binary from the target minor", {
+  v <- seed_verdict(make_tarball(), "R6", "2.6.1", "4.5", portable)
+  expect_true(v$seed)
+  expect_identical(v$reason, "seeded")
+})
+
+test_that("seed_verdict refuses what a fresh build would not publish flat", {
+  verdict <- function(tarball, minor = "4.5", abi = portable) {
+    seed_verdict(tarball, "R6", "2.6.1", minor, abi)$reason
+  }
+  expect_identical(verdict(make_tarball(built = NA)), "source")
+  expect_identical(
+    verdict(make_tarball(
+      built = "R 4.5.3; x86_64-pc-linux-musl; 2026-09-01; unix"
+    )),
+    "compiled"
+  )
+  expect_identical(verdict(make_tarball(libs = TRUE)), "compiled")
+  expect_identical(verdict(make_tarball(), minor = "4.6"), "r-minor")
+  expect_identical(verdict(make_tarball(version = "2.6.0")), "mismatch")
+  # Unknown is a refusal, as is a verdict that some minor cannot load it.
+  expect_identical(
+    verdict(make_tarball(), abi = function(t) list(inspected = FALSE)),
+    "abi"
+  )
+  expect_identical(
+    verdict(
+      make_tarball(),
+      abi = function(t) list(inspected = TRUE, sensitive = TRUE)
+    ),
+    "abi"
+  )
+  junk <- tempfile(fileext = ".tar.gz")
+  writeLines("not a tarball", junk)
+  expect_identical(verdict(junk), "unreadable")
+})
+
+test_that("seed_candidates keeps only uncopied pure-R binaries in scope", {
+  donor <- c(
+    "R6_2.6.1.tar.gz",
+    "Archive/R6/R6_2.5.1.tar.gz",
+    "curl_7.0.0.tar.gz",
+    "4.6/R6_2.6.1.tar.gz",
+    "glue_1.8.0.tar.gz",
+    "gone_1.0.tar.gz",
+    "done_1.0.tar.gz",
+    "skipme_1.0.tar.gz",
+    "PACKAGES.tar.gz"
+  )
+  etags <- c("a", "b", "c", "d", "c0ffee", "e", "f", "g", "h")
+  out <- seed_candidates(
+    donor,
+    etags,
+    target_keys = c("done_1.0.tar.gz", "4.6/R6_2.5.1.tar.gz"),
+    release_pkgs = c("R6", "curl", "glue", "done", "skipme"),
+    compiled_pkgs = "curl",
+    cran_md5 = c(glue_1.8.0 = "c0ffee", R6_2.6.1 = "zzz"),
+    exclude = "skipme"
+  )
+  expect_identical(out$key, c("R6_2.6.1.tar.gz", "Archive/R6/R6_2.5.1.tar.gz"))
+  expect_identical(out$package, c("R6", "R6"))
+  expect_identical(out$version, c("2.6.1", "2.5.1"))
+})
+
+test_that("seed_candidates copies a version listed twice once, flat first", {
+  out <- seed_candidates(
+    c("Archive/R6/R6_2.6.1.tar.gz", "R6_2.6.1.tar.gz"),
+    c("a", "a"),
+    character(),
+    "R6",
+    character(),
+    character()
+  )
+  expect_identical(out$key, "R6_2.6.1.tar.gz")
+})
+
+test_that("parse_built separates the R version from the platform", {
+  b <- parse_built("R 4.5.3; ; 2026-09-01 10:00:00 UTC; unix")
+  expect_identical(b$r_version, "4.5.3")
+  expect_identical(b$platform, "")
+  expect_identical(
+    parse_built("R 4.4.3; aarch64-alpine-linux-musl; 2026; unix")$platform,
+    "aarch64-alpine-linux-musl"
+  )
+})
+
+test_that("codename_family groups versions of one distribution", {
+  expect_identical(codename_family("alpine325"), "alpine")
+  expect_identical(codename_family("alpine324"), codename_family("alpine325"))
+  expect_false(identical(
+    codename_family("rhel9"),
+    codename_family("alpine324")
+  ))
+})