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.

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")
+  ))
+})