rpc: add Monad + MegaETH chains

[Flotapponnier]

Summary

• Monad mainnet (143): monad-official (rpc.monad.xyz), dRPC, Tenderly, bloXroute, OnFinality — 5 keyless providers, all live-verified (chainId + anti-cache probe)
• MegaETH mainnet (4326): megaeth-official, 1RPC, dRPC, Tenderly — 4 keyless providers, verified
• Probe load 1/30s/region stays far under every documented rate limit (lowest: rpc1.monad.xyz 15 rps, not included; official 25 rps)

Not in this PR

• Per-chain pages (/benchmarks/monad-rpc, megaeth-rpc) + /rpc hub inclusion: after 48h of verified clean data, per review rule
• l1-finality / l2-block-time additions: separate PR