fix: DYNAMIC_SERVER_USAGE 500 on draft bench pages + retire perp-oi

[Flotapponnier]

Summary

• Remove noStore()/unstable_noStore() calls that were made after unstable_cache had already been invoked in the same render — Next.js 15 throws DYNAMIC_SERVER_USAGE when this happens, causing 500 on all draft/awaiting bench pages
• Removed from both src/lib/spec.ts (inside AllBenchmarksDraftError handler) and src/app/benchmarks/[slug]/page.tsx (the if (isAwaiting) noStore() block)
• Side effect: awaiting benches now stay in ISR for up to 60s instead of always re-rendering — acceptable tradeoff vs. 500 errors
• Also retires perp-open-interest bench (structurally biased multi-chain OI aggregation)

Test plan

• Check a draft bench page like /benchmarks/perp-exit-custody or /benchmarks/swellchain-rpc returns 200 on staging
• Check a live bench page still returns 200
• No TypeScript errors (next build clean)