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/r-minor-helpers.R

@@ -31,3 +31,33 @@ parse_build_args <- function(args) {
     ncpus = as.integer(pos[3L])
   )
 }
+
+# The `single_builds` platform name for a bincraft codename, e.g. alpine324 ->
+# "alpine-324", rhel9 -> "redhat-9". NA for a codename this repo does not build.
+codename_platform <- function(codename) {
+  ubuntu <- c(jammy = "2204", noble = "2404", resolute = "2604")
+  if (codename %in% names(ubuntu)) {
+    return(paste0("ubuntu-", ubuntu[[codename]]))
+  }
+  if (grepl("^rhel[0-9]+$", codename)) {
+    return(sub("^rhel", "redhat-", codename))
+  }
+  if (grepl("^alpine[0-9]+$", codename)) {
+    return(sub("^alpine", "alpine-", codename))
+  }
+  NA_character_
+}
+
+# The wall-clock deadline a build loop must stop at, from BUILD_DEADLINE (epoch
+# seconds). The pipeline sets it once per step, so every pass in the step shares
+# one budget and the steps after the builds (archiving, the index upload) still
+# get to run before Crow's timeout kills the step. Unset or unparsable means no
+# deadline.
+build_deadline <- function(value = Sys.getenv("BUILD_DEADLINE", unset = "")) {
+  deadline <- suppressWarnings(as.numeric(value))
+  if (length(deadline) != 1L || is.na(deadline)) Inf else deadline
+}
+
+deadline_reached <- function(deadline, now = as.numeric(Sys.time())) {
+  now >= deadline
+}