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/build-all.R

@@ -1,6 +1,7 @@
 sink(stdout(), type = "message")
 options(crayon.enabled = TRUE, future.globals.onReference = NULL)
 source(file.path("local", "r-minor-helpers.R"))
+source(file.path("local", "rebuild-missing-helpers.R"))
 
 args <- commandArgs(trailingOnly = TRUE)
 parsed <- parse_build_args(args)
@@ -104,9 +105,10 @@ sprintf(
   sensitive_only
 )
 
-# Split into chunks for this worker
-chunks <- split(pkgs, cut(seq_len(nrow(pkgs)), split_into, labels = FALSE))
-chunk <- chunks[[split_index]]
+# Interleave rather than cut the alphabetical list into contiguous blocks: build
+# cost clusters by name (Rcpp*, rstan*), and contiguous quarters of the last
+# full rebuild ran anywhere from 0.1h to 28h.
+chunk <- pkgs[shard_slice(seq_len(nrow(pkgs)), split_into, split_index), ]
 sprintf("# of package versions for this job: %s", nrow(chunk))
 
 # Exclude known problematic packages (single source of truth)
@@ -126,23 +128,7 @@ chunk <- chunk[!chunk$Package %in% exclude, ]
 # environment, so Sys.getenv() would return "" and this pre-filter would query
 # platform "-" and skip nothing.
 codename <- bincraft::set_codename(NULL)
-platform <- switch(
-  codename,
-  jammy = "ubuntu-2204",
-  noble = "ubuntu-2404",
-  resolute = "ubuntu-2604",
-  rhel10 = "redhat-10",
-  rhel9 = "redhat-9",
-  rhel8 = "redhat-8",
-  alpine320 = "alpine-320",
-  alpine321 = "alpine-321",
-  alpine322 = "alpine-322",
-  alpine323 = "alpine-323",
-  alpine324 = "alpine-324",
-  alpine325 = "alpine-325",
-  alpine326 = "alpine-326",
-  NA_character_
-)
+platform <- codename_platform(codename)
 local_machine <- Sys.info()[["machine"]]
 arch <- if (grepl("arm64|aarch64", local_machine)) "arm64" else "amd64"
 con <- DBI::dbConnect(
@@ -232,35 +218,41 @@ sprintf(
   length(s3_cache_paths)
 )
 
+deadline <- build_deadline()
 n <- nrow(chunk)
-mapply(
-  function(pkg, ver, sens, i) {
-    cat(sprintf("[%d/%d] %s_%s (r_minor_sensitive=%s)\n", i, n, pkg, ver, sens))
-    bincraft::build_binary_package(
-      pkg,
-      tag = ver,
-      is_r_minor_sensitive = isTRUE(sens),
-      s3_endpoint = "https://s3.eu-central-003.backblazeb2.com",
-      s3_region = "eu-central-003",
-      s3_bucket = "devxy-rpkgs-binaries",
-      s3_access_key_id = Sys.getenv("B2_S3_ACCESS_KEY"),
-      s3_secret_access_key = Sys.getenv("B2_S3_SECRET_KEY"),
-      s3_package_cache = s3_cache,
-      metadata_db_host = "r-binaries.devxy.io",
-      metadata_db_name = "build_metadata",
-      metadata_db_table = "single_builds",
-      metadata_db_user = "rpkgs",
-      metadata_db_password = Sys.getenv("PGPASS"),
-      metadata_db_sslmode = "verify-full",
-      metadata_db_port = 15432,
-      archive = TRUE,
-      patches = "local/patches",
-      upload = TRUE,
-      store_build_metadata = TRUE
-    )
-  },
-  chunk$Package,
-  chunk$Version,
-  chunk$r_minor_sensitive,
-  seq_len(n)
-)
+for (i in seq_len(n)) {
+  if (deadline_reached(deadline)) {
+    cat(sprintf(
+      "Build deadline reached after %d/%d package versions; stopping cleanly. The next run resumes from the metadata DB.\n",
+      i - 1L,
+      n
+    ))
+    break
+  }
+  pkg <- chunk$Package[i]
+  ver <- chunk$Version[i]
+  sens <- chunk$r_minor_sensitive[i]
+  cat(sprintf("[%d/%d] %s_%s (r_minor_sensitive=%s)\n", i, n, pkg, ver, sens))
+  bincraft::build_binary_package(
+    pkg,
+    tag = ver,
+    is_r_minor_sensitive = isTRUE(sens),
+    s3_endpoint = "https://s3.eu-central-003.backblazeb2.com",
+    s3_region = "eu-central-003",
+    s3_bucket = "devxy-rpkgs-binaries",
+    s3_access_key_id = Sys.getenv("B2_S3_ACCESS_KEY"),
+    s3_secret_access_key = Sys.getenv("B2_S3_SECRET_KEY"),
+    s3_package_cache = s3_cache,
+    metadata_db_host = "r-binaries.devxy.io",
+    metadata_db_name = "build_metadata",
+    metadata_db_table = "single_builds",
+    metadata_db_user = "rpkgs",
+    metadata_db_password = Sys.getenv("PGPASS"),
+    metadata_db_sslmode = "verify-full",
+    metadata_db_port = 15432,
+    archive = TRUE,
+    patches = "local/patches",
+    upload = TRUE,
+    store_build_metadata = TRUE
+  )
+}