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 locating the engine condition evaluation and the SDK getFlags path, then inspect engine-test-data for the flag dependency cases. Implement support for $.flags conditions and segment passing while preserving per-evaluation isolation and avoiding cost when unused. Done means the bumped engine-test-data cases pass and identity-less evaluations resolve $.flags and $.environment conditions.

Written by the indexing model from the issue text.