docs: Arcscan (Blockscout) smart contract source verification undocumented — hardhat-verify customChains configuration missing

[osr21]

Summary

When deploying Solidity contracts to Arc Testnet (and presumably Mainnet), Arcscan displays:

"Contract not verified automatically. Please verify manually."

There is no official documentation in this repo, in the Arc docs, or anywhere on arc.io that explains:

1. Which hardhat-verify plugin to use
2. What the Blockscout API URL is for Arc Testnet / Mainnet
3. That a dummy API key is required (Blockscout ignores API keys but hardhat-verify rejects an empty string)
4. That sourcify: { enabled: false } must be set to prevent conflicting verification attempts

---

Discovered while building ArcPay on Arc Testnet

After deploying three Solidity contracts (InvoiceRegistry, PaymentEscrow, InvoiceNFT) to Arc Testnet and seeing the "not verified" banner on Arcscan, I had to reverse-engineer the following working configuration by trial and error:

// hardhat.config.ts
import "@nomicfoundation/hardhat-verify";

const config: HardhatUserConfig = {
  // ... solidity, networks ...
  etherscan: {
    apiKey: {
      arc: "empty",          // Blockscout ignores the key but rejects an empty string
      arcMainnet: "empty",
    },
    customChains: [
      {
        network: "arc",
        chainId: 5042002,     // Arc Testnet
        urls: {
          apiURL: "https://testnet.arcscan.app/api",
          browserURL: "https://testnet.arcscan.app",
        },
      },
      {
        network: "arcMainnet",
        chainId: 4564,        // Arc Mainnet
        urls: {
          apiURL: "https://arcscan.app/api",
          browserURL: "https://arcscan.app",
        },
      },
    ],
  },
  sourcify: {
    enabled: false,   // Prevents duplicate/conflicting verification via Sourcify
  },
};

Then verify with:

npx hardhat verify --network arc <CONTRACT_ADDRESS> <CONSTRUCTOR_ARG1> <CONSTRUCTOR_ARG2>

---

Impact

Every developer deploying a dApp on Arc Network hits this friction point. The arc-node repo already contains hardhat.config.ts but has no etherscan or sourcify block. The "verify manually" banner on Arcscan gives no hint of where to start.

---

Suggested fix

1. Add the etherscan.customChains block above to the repo's hardhat.config.ts as a reference
2. Add a docs/contract-verification.md section walking through the hardhat-verify flow for Arc Testnet and Mainnet
3. Optionally: update Arcscan's "verify manually" page to link to this documentation

---

Environment: Arc Testnet (chainId 5042002), @nomicfoundation/hardhat-verify v3, Hardhat 2.28, Solidity 0.8.20

[osr21]

✅ Working configuration confirmed

Here's the exact setup verified against Arc Testnet + Mainnet:

hardhat.config.ts snippet:

import "@nomicfoundation/hardhat-verify";

etherscan: {
  apiKey: {
    arc: "empty",          // Blockscout ignores the key — any non-empty string works
    arcMainnet: "empty",
  },
  customChains: [
    {
      network: "arc",
      chainId: 5042002,
      urls: {
        apiURL: "https://testnet.arcscan.app/api",
        browserURL: "https://testnet.arcscan.app",
      },
    },
    {
      network: "arcMainnet",
      chainId: 5042,
      urls: {
        apiURL: "https://arcscan.app/api",
        browserURL: "https://arcscan.app",
      },
    },
  ],
},
sourcify: { enabled: false },  // Must be set — prevents conflicting Sourcify attempts

Verify:

npx hardhat verify --network arc 0xYourContractAddress constructorArg1 constructorArg2

Foundry equivalent (foundry.toml):

[etherscan]
arc = { key = "empty", url = "https://testnet.arcscan.app/api" }
arcMainnet = { key = "empty", url = "https://arcscan.app/api" }

forge verify-contract <ADDRESS> src/MyContract.sol:MyContract \
  --chain-id 5042002 --etherscan-api-key empty \
  --verifier-url https://testnet.arcscan.app/api

Important gotcha: the compiler version, optimizer settings, and EVM version target must match the deployed bytecode exactly. A common failure mode is that hardhat.config.ts in this repo uses evmVersion: 'prague' — if you're verifying contracts compiled with that setting, make sure the same evmVersion is set in your verification config.

Full reference docs added to PR #79: docs/contract-verification.md