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/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
+}