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.

Start by tracing getFlags and how it passes segments to the engine, then inspect the engine-test-data dependency and its flag-dependency cases. Verify that $.flags and $.environment resolve during identity-less evaluation, that evaluations without $.flags conditions avoid extra cost, and that resolved flags are isolated between concurrent evaluations.

Written by the indexing model from the issue text.