feat(rpc): per-chain RPC cluster — 10 first-class benches (044-053)

[Flotapponnier]

What

Promotes the 10 per-chain RPC leaderboards from variant pages (/benchmarks/rpc-capabilities/{chain}) to first-class benchmarks with their own YAML, editorial, FAQ and region dimension:

ethereum-rpc (044, 9 providers) · arbitrum-rpc (045, 8) · base-rpc (046, 6) · optimism-rpc (047, 6) · avalanche-rpc (048, 6) · bnb-rpc (049, 5) · polygon-rpc (050, 5) · linea-rpc (051, 4) · scroll-rpc (052, 4) · mantle-rpc (053, 4)

Why

• SEO: each page targets its own head query ("fastest polygon rpc") with full editorial (chain-specific findings + 5 FAQ each), OG cards, catalog presence — vs a thin variant page.
• Badges: 10 per-chain drop journal framing #1 slots for the provider outreach program instead of one global drop journal framing #1.
• Docs push: chain docs PRs get a dedicated URL anchor (/benchmarks/polygon-rpc).

How

• Generator script (scripts/generate-rpc-chain-benches.py) reads provider metadata from the parent and bakes chain="X" into every PromQL selector. Region stays a dimension (all/us-east/eu-west/sgp) with rank_matrix_query per bench for scoped badges.
• Provider matrix per chain verified against live Prom (2026-07-03).
• Editorial is chain-specific (no template duplication): Cloudflare-eth trap on Ethereum, best-effort foundation tails on Arbitrum/Optimism, dataseed regional extremes on BNB, Linea/Scroll controlled-experiment framing, etc.
• Parent rpc-capabilities keeps the cross-chain role; its per_chain_explainer block is removed and the old variant URLs 301 to the new benches (next.config.ts).

Validation

• npx tsx scripts/validate-specs.ts → all 10 pass (only pre-existing perp-open-interest error remains)
• dry-run-spec polygon-rpc against prod Prom → live data for all 5 providers (dRPC 22ms leader)
• Titles 26-31 chars (≤60 effective with suffix)
• tsc clean

Test plan (preview)

• /benchmarks/polygon-rpc renders leaderboard + region tabs
• /benchmarks/rpc-capabilities/polygon → 301 → /benchmarks/polygon-rpc
• /benchmarks index lists the 10 new benches under RPCs
• sitemap contains the 10 new URLs, no longer the variant URLs