docs: add Arc Relay Bridge ecosystem documentation

[osr21]

Summary

Adds developer documentation for Arc Testnet from real-world DApp integration experience.
All findings come from building Arc Relay Bridge —
a cross-chain USDC bridge on Circle's CCTP V2 — and are verified against the live testnet.

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Files added / updated

File	Description
docs/ecosystem/arc-relay-bridge.md	Full ecosystem reference for the Arc Relay Bridge project
docs/cctp-v2.md	Arc-specific CCTP V2 integration guide — nonce encoding, gas workarounds, attestation timing, RPC gotchas
docs/dapp-development.md	New — practical DApp developer guide: MetaMask integration, RPC CORS, USDC decimals, gas quirks, eth_getLogs pagination, EIP-2612 permit
docs/contract-verification.md	New — step-by-step contract verification on ArcScan (Blockscout) for both Hardhat and Foundry

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Why this is useful for Arc developers

docs/dapp-development.md documents every friction point encountered when building a
browser DApp on Arc Testnet:

• wallet_switchEthereumChain fails silently — must use wallet_addEthereumChain unconditionally (tracks #89)
• Public RPC missing CORS headers — use arc-testnet.drpc.org in browser DApps (tracks #90)
• USDC has 6 decimals for ERC-20 ops, 18 decimals for gas — undocumented, causes silent value bugs (tracks #91)
• eth_estimateGas unreliable — always supply explicit gasLimit: 600_000n (tracks #80)
• Gas price stale baseline — apply ≥30% premium for back-to-back transactions (tracks #87)
• eth_getLogs times out for wide block ranges — paginate at ≤10,000 blocks (tracks #83)
• EIP-2612 permit works on Arc USDC — working ethers.js v6 snippet included (tracks #93)
• MetaMask nativeCurrency must use symbol: "ETH" / decimals: 18 despite USDC as gas (tracks #95)
• wallet_watchAsset required to show USDC in MetaMask token list (tracks #96)

docs/contract-verification.md documents the entire ArcScan verification flow that
currently requires reverse-engineering:

• Blockscout API URL (testnet.arcscan.app/api)
• Hardhat etherscan.customChains block with dummy API key
• Foundry foundry.toml equivalent
• Manual verification via ArcScan UI
• Blockscout REST API for programmatic verification
• The bytecode-must-match-exactly constraint and why source can become unrecoverable
• Testnet and Mainnet entries (tracks #84)

docs/cctp-v2.md covers Arc-specific CCTP V2 behaviors not visible in Circle's docs:

• Arc nonce encoding (bytes32 always zero, not uint64)
• 7-param depositForBurn requirement (selector 0x8e0250ee)
• minFinalityThreshold: 2000 (finalized) for Arc vs 1000 (safe) for other chains
• Gas estimation bug + waitWithRetry pattern for polling
• RPC endpoint selection for browser DApps

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Built with: React 19 + Vite + ethers.js v6 + Circle Iris API sandbox