Add USDC EIP-2612 Permit documentation

usdc-permit.md

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+# USDC EIP-2612 Permit
+
+Arc Testnet USDC supports EIP-2612 `permit`, allowing a token allowance to be approved with an off-chain signature instead of a separate on-chain `approve` transaction.
+
+## Arc Testnet USDC
+
+- Chain ID: `5042002`
+- USDC contract: `0x3600000000000000000000000000000000000000`
+
+## EIP-712 Domain
+
+When signing an EIP-2612 permit for Arc Testnet USDC, use:
+
+```ts
+const domain = {
+  name: "USDC",
+  version: "2",
+  chainId: 5042002,
+  verifyingContract: USDC_ADDRESS,
+};
+const types = {
+  Permit: [
+    { name: "owner", type: "address" },
+    { name: "spender", type: "address" },
+    { name: "value", type: "uint256" },
+    { name: "nonce", type: "uint256" },
+    { name: "deadline", type: "uint256" },
+  ],
+};
+const nonce = await publicClient.readContract({
+  address: USDC_ADDRESS,
+  abi: erc20PermitAbi,
+  functionName: "nonces",
+  args: [userAddress],
+});
+
+const deadline = BigInt(Math.floor(Date.now() / 1000) + 3600);
+
+const signature = await walletClient.signTypedData({
+  domain: {
+    name: "USDC",
+    version: "2",
+    chainId: 5042002n,
+    verifyingContract: USDC_ADDRESS,
+  },
+  types: {
+    Permit: [
+      { name: "owner", type: "address" },
+      { name: "spender", type: "address" },
+      { name: "value", type: "uint256" },
+      { name: "nonce", type: "uint256" },
+      { name: "deadline", type: "uint256" },
+    ],
+  },
+  primaryType: "Permit",
+  message: {
+    owner: userAddress,
+    spender: relayerAddress,
+    value: amount,
+    nonce,
+    deadline,
+  },
+});
+```