docs(contracts): document Arcscan Etherscan-compatible contract verification

contracts/README.md

@@ -12,6 +12,46 @@ The genesis builder (`contracts/scripts/ArtifactHelper.s.sol`), all CREATE2-sens
 tests (`tests/localdev/genesis.test.ts`) all read from `contracts/out/forge/`. 
 Hardhat's compile output is **not** consumed for any CREATE2-sensitive path.
 
+## Verifying contracts on Arcscan
+
+Arc Testnet's explorer ([testnet.arcscan.app](https://testnet.arcscan.app)) exposes an
+**Etherscan-compatible** contract verification API at `https://testnet.arcscan.app/api`.
+Blockscout-native field names (`addressHash`, `name`, `contractSourceCode`, …) are **not**
+accepted and return a misleading `Missing codeformat field` error even when `codeformat` is
+present — that is the failure mode reported in #210. Use Etherscan V1 field names
+(`contractaddress`, `contractname`, `sourceCode`, `codeformat`, …) or a tool that already
+speaks that dialect.
+
+### Foundry
+
+Arcscan ignores the API key value but Foundry still requires a non-empty key:
+
+```bash
+export ARC_TESTNET_RPC_URL=https://rpc.testnet.arc.io
+forge verify-contract <address> path/to/Contract.sol:ContractName \
+  --chain 5042002 \
+  --verifier custom \
+  --verifier-url https://testnet.arcscan.app/api \
+  --verifier-api-key empty \
+  --watch
+```
+
+Pass constructor args with `--constructor-args` / `--constructor-args-path` when needed.
+`foundry.toml` also registers an `arc_testnet` Etherscan entry so `--chain arc-testnet` (or `5042002`) works
+once `ETHERSCAN_API_KEY` (any non-empty string) is set.
+
+### Hardhat
+
+```bash
+export ARC_TESTNET_RPC_URL=https://rpc.testnet.arc.io
+npx hardhat verify --network testnet <address> [constructorArgs...]
+```
+
+This requires `@nomicfoundation/hardhat-verify` with an Arc Testnet `customChains` /
+`etherscan` entry for chain ID `5042002` pointing at `https://testnet.arcscan.app/api`
+(see open wiring in related PRs if it is not yet on `main`). Disable Sourcify so it does not
+race Arcscan.
+
 ## Foundry
 
 **Foundry is a blazing fast, portable and modular toolkit for Ethereum application development written in Rust.**

foundry.toml

@@ -30,3 +30,8 @@ fs_permissions = [
 [lint]
 ignore = ["contracts/test/scripts/*.t.sol"]
 # See more config options https://github.com/foundry-rs/foundry/blob/master/crates/config/README.md#all-options
+
+# Arcscan is Blockscout-backed but speaks the Etherscan V1 verify API (see #210).
+# A non-empty key is required by Foundry; Arcscan ignores the value.
+[etherscan]
+arc_testnet = { key = "${ETHERSCAN_API_KEY}", chain = 5042002, url = "https://testnet.arcscan.app/api" }