docs: add MetaMask integration guide with USDC token setup

[cryptoasuran]

Summary

Adds comprehensive MetaMask integration guide for Arc Testnet, including the critical wallet_watchAsset call to register USDC.

Problem

Issue #97 identified that there's no documented way to add Arc Testnet USDC to MetaMask's token list. Without this:

• Users see no USDC balance after receiving tokens
• Users assume transactions failed
• DApps appear broken

Solution

New guide at docs/metamask-integration.md covering:

1. Network setup with wallet_addEthereumChain
2. USDC token registration with wallet_watchAsset (the missing piece)
3. Complete onboarding flow combining both steps
4. Contract addresses and chain configuration
5. Why this matters section explaining the impact

Changes

• ✅ New file: docs/metamask-integration.md
• ✅ Updated README.md to link to new guide
• ✅ Complete TypeScript code examples
• ✅ Addresses issue reproduction steps

Testing

Code examples follow MetaMask's official API documentation:

• wallet_watchAsset
• wallet_addEthereumChain

Impact

Every DApp on Arc Testnet that involves USDC transfers benefits from this documentation.

Fixes #97

[osr21]

Great guide — this addresses the exact pain point in #97.

One more MetaMask behaviour on Arc Testnet that's worth documenting alongside the wallet_watchAsset call: wallet_switchEthereumChain fails silently (tracked in #89).

Even after the network has been added, calling wallet_switchEthereumChain for Arc Testnet either resolves without switching or throws a 4902 even when the chain exists. The only reliable pattern is to always use wallet_addEthereumChain, which acts as both "add if missing" and "switch if present":

// ❌ Unreliable on Arc Testnet — may resolve without actually switching
await window.ethereum.request({
  method: 'wallet_switchEthereumChain',
  params: [{ chainId: '0x4CEF52' }],
});

// ✅ Reliable — works whether network is already added or not
await window.ethereum.request({
  method: 'wallet_addEthereumChain',
  params: [{
    chainId: '0x4CEF52',
    chainName: 'Arc Testnet',
    nativeCurrency: { name: 'Ether', symbol: 'ETH', decimals: 18 },
    rpcUrls: ['https://rpc.drpc.testnet.arc.network'],
    blockExplorerUrls: ['https://testnet.arcscan.app'],
  }],
});

Note the nativeCurrency — Arc's gas is paid in USDC but MetaMask requires symbol: 'ETH' and decimals: 18 to accept the chain config (documented in #95). Using the actual USDC values causes MetaMask to reject the wallet_addEthereumChain call entirely.

Also: the rpcUrls entry matters here too — rpc.testnet.arc.network has CORS issues for browser DApps (#90), so rpc.drpc.testnet.arc.network is the reliable public endpoint to point users at.