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

[Flotapponnier]

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, category, 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. The aggregation logic is extracted into a pure
aggregateBenchmarks function so the test does not need Prometheus or
unstable_cache.

Verification

Before:

• /api/citable: every bench reports status draft, value null, leader null
• /api/stat/bridge-quote-latency: status live, value 298.7, leader Mobula

After (locally, via the regression test): aggregator throws
AllBenchmarksDraftError when every loader fails or returns undefined,
returns the partial list when at least one bench is live, never returns
a stable all-draft set.

Test plan

• pnpm typecheck
• pnpm lint (no new warnings)
• bun test src/lib/ (83 pass)
• pnpm build (no crash on the all-draft path either)
• After merge to dev: curl preview /api/citable, confirm live values
• Cherry-pick to main once preview looks healthy