Task 5 - EngineVault 核心

Scope:
- ERC-4626 核心接口
- cycle() 主循环
- _computeTargetAllocation / _rebalanceAssets / _rebalanceHedge
- bounty gasPrice cap + buffer cap

Acceptance:
- fork 用例 A/B 通过
- CycleExecuted / RegimeSwitched 事件完整