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.

先定位 getFlags 和引擎集成，然后检查 engine-test-data 依赖及其 flag 依赖案例。验证 $.flags 条件在无身份评估中能够正常工作，不包含此类条件的环境不会产生每次评估的开销，并且已解析的 flag 在并发评估之间保持隔离。

由索引模型根据 Issue 内容生成。