hotfix(citable): stop poisoning aggregator cache with all-draft set

[Flotapponnier]

Cherry-pick of #607 to main as a hotfix.

Fixes /api/citable returning all benchmarks as draft when /api/stat/
for the same bench returns live data. Root cause: loadAllBenchmarksCached
was returning an all-draft set whenever per-bench fetches threw at cold
start, and the cache then served that poisoned set for the rest of the
revalidate window.

This change restores the throw-on-all-draft behaviour at the cache
boundary so unstable_cache keeps the previous good value during a Prom
blackout. The known call sites are wrapped explicitly: API endpoints and
feeds (citable, llm-context, llms.txt, rss.xml) return 503 with a
retry-after hint, pages and shared loaders (home, benchmarks index,
bench slug, mcp page, chains, providers) use a new getBenchmarksSafe
wrapper that renders draft placeholders, and the sitemap already uses
safeLoad which catches and falls back.

Cache key bumped from v12 to v13 to flush any poisoned snapshot already
stored in Upstash KV.

Adds a regression test that asserts the aggregator never returns a
stable all-draft list.

Cherry-pick notes

Some conflicts resolved against main's slightly older surface:

• src/app/benchmarks/category/[cat]/page.tsx is deleted on main, dropped
  from the cherry-pick.
• src/app/api/citable/route.ts does not have citeBundle on main, kept
  the existing field set, only added the AllBenchmarksDraftError import
  and the try/catch + 503.
• src/lib/providers.ts isAll import not needed on main, dropped.
• src/app/benchmarks/[slug]/page.tsx kept main's lucide-react + import
  surface, only switched the loader to getBenchmarksSafe.

Test plan

• pnpm typecheck
• pnpm lint (no new warnings vs main baseline)
• bun test src/lib/ (34 pass on main worktree, including the new regression tests)
• pnpm build
• After merge: vercel --prod from main checkout, then curl prod /api/citable