feat: add MegaETH mainnet and testnet support - #41

Summary

Adds support for the MegaETH chain (resolves #23).

Two networks are added because viem 2.39.3 (the version pinned by this repo) exports neither of them:

• MegaETH mainnet — chain id 4326
• MegaETH Testnet — chain id 6343

Note: viem's built-in megaethTestnet points at chain id 6342, which the chain registry (chainid.network / ethereum-lists/chains) now marks as deprecated. So both networks are defined locally with defineChain using the current registry values.

Changes

• src/core/chains.ts
  • define megaeth and megaethTestnet via defineChain
  • register both in chainMap, networkNameMap, and rpcUrlMap
• README.md
  • add MegaETH to the mainnet list and MegaETH Testnet to the testnet list

Verification

• bunx tsc --noEmit passes
• bun run build passes
• Confirmed both RPC endpoints respond with the expected chain ids:
  • https://mainnet.megaeth.com/rpc -> 0x10e6 (4326)
  • https://carrot.megaeth.com/rpc -> 0x18c7 (6343)