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-newer-r.R

@@ -1,5 +1,6 @@
 source(file.path("local", "newer-r-helpers.R"))
 source(file.path("local", "rebuild-missing-helpers.R"))
+source(file.path("local", "r-minor-helpers.R"))
 
 args <- commandArgs(trailingOnly = TRUE)
 pos <- args[!startsWith(args, "--")]
@@ -117,7 +118,16 @@ if ("--dry-run" %in% args) {
   ))
 
   options(crayon.enabled = TRUE, Ncpus = 4, future.globals.onReference = NULL)
+  deadline <- build_deadline()
   for (i in seq_len(nrow(pkgs))) {
+    if (deadline_reached(deadline)) {
+      message(sprintf(
+        "Build deadline reached after %d/%d package(s); stopping cleanly.",
+        i - 1L,
+        nrow(pkgs)
+      ))
+      break
+    }
     message(sprintf(
       "[%d/%d] %s %s",
       i,