mev-protect-rpc: chain dimension + 4 new gateways + consistency answer

[Flotapponnier]

Summary

• Harness: chain label on all metrics, one row per (provider, chain), rows probed in parallel (each gateway still sees 1 req/1.5s). Cohort verified live: Ethereum adds bloXroute Protect + BlockSec; BSC adds Blink, bloXroute, 48 Club, PancakeSwap MEV Guard, BlockSec; Base has Blink only (sole keyless MEV-protect gateway on Base today)
• Spec: chain dimension tabs, Chains covered panel, 4 new provider blocks, methodology + findings updated (Blink is the only gateway live on all 3 chains)
• New answers page: which-mev-protection-rpc-is-the-most-consistent (p99 vs p50 lens, Blink flattest tail)
• Deployed to the 3 Railway probe regions

All endpoints live-verified before inclusion (eth_chainId probes). Alchemy MEV Protect + GetBlock excluded (key-gated), base.rpc.blxrbdn.com excluded (not documented as Protect).