Implement YieldSave vault MVP and Aave testnet coverage - #1

• implement the YieldSave vault MVP with share accounting and yield-only fee logic
• add Foundry config, deploy scripts, and network address templates
• add mock-based scenario tests and optional Base Sepolia fork integration tests

Testing

• forge test
• BASE_SEPOLIA_RPC_URL= forge test --match-path test/fork/BaseSepoliaIntegration.t.sol -vv"