docs: add ERC-4337 developer guide for Arc Testnet

Covers the three Arc-specific constraints:
- Compile with evmVersion: "paris" (no PUSH0 on Arc Testnet)
- Keep entryPoint as constant to avoid 0x60c0 silent revert
- No nonReentrant on validatePaymasterUserOp (ERC-7562)

Also documents:
- USDC-as-gas decimal handling (18-dec native vs 6-dec ERC-20)
- EntryPoint v0.7 address and Pimlico bundler setup
- USDC Paymaster pattern with reservation system
- Deterministic finality and CCTP minFinalityThreshold guidance
- Full worked example: gasless USDC bridge burn via CCTP V2
- Debugging checklist with all known silent failure modes

Author: osr21