Support `$.flags` context values in segment conditions

Support `$.flags..` conditions.

### Acceptance criteria

Engine:

- `engine-test-data` is bumped to the tag containing the flag dependency cases, and they pass.
- An environment with no `$.flags` conditions gains no per-evaluation cost.
- Resolved flags are scoped to a single evaluation, never shared between concurrent evaluations.

SDK:

- `getFlags` passes segments to the engine so identity-less evaluation resolves `$.flags` and `$.environment` conditions.

首先跟踪 SDK getFlags 进入 engine，并检查 segments 和 per-evaluation context 是如何传递的。使用包含 flag dependency cases 的 tag engine-test-data 作为第一个验证点；完成意味着 $.flags 条件适用于无 identity 的 evaluation，不包含它们的 environments 不会产生额外的 per-evaluation 成本，并发 evaluations 不会共享已解析的 flags。

由索引模型根据 Issue 内容生成。