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/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,