fix(build): render bench pages on demand

[Flotapponnier]

Summary

• The fix(perf): fetch bench variants on demand, stop embedding the full matrix #399 staging deploy failed at build: /benchmarks/pm-data-freshness export timed out 3×60s (CI runner DNS throttling under the hundreds of Prom lookups the 25+ prerendered bench pages fire)
• Bench pages now render on demand like /products: empty generateStaticParams, first hit per slug per deploy renders in a few seconds (cheap now that the variant matrix isn't embedded), then ISR + CDN
• Deploys no longer depend on Prom availability at build time at all

Test plan

• tsc, bun test
• Staging deploy goes green; /api/bench/[slug]/variant responds 200; bench TTFB sub-second once warm