fix(perf): fetch bench variants on demand, stop embedding the full matrix

[Flotapponnier]

Summary

• Root cause of the 30-60s bench page loads (measured 52s TTFB): every ISR regeneration awaited the full chain×region×kind variant matrix (39 full provider loads on rpc-capabilities) before responding, and build logs show the related data-cache items exceed Vercel's 2MB cap so they never cache
• The page now ships ONLY the aggregate view; BenchmarkBody fetches a variant from the new /api/bench/[slug]/variant route when a tab is flipped (validated filters, editorial override preserved, s-maxage=60 + per-variant unstable_cache dedupes across users)
• While a variant loads the aggregate keeps rendering, so tab flips never blank; failed fetches keep the aggregate

Test plan

• tsc, bun test
• Staging: TTFB sub-second on bench pages after warm-up, tab flips populate chain/region data within ~500ms, tab summary chips fill as variants load