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 with the SDK's getFlags path and the engine integration, then inspect how segments and evaluation context are passed. Run the engine-test-data dependency cases, including flag and environment conditions. Done means identity-less evaluations resolve $.flags and $.environment, evaluations without flag conditions avoid per-evaluation cost, and concurrent evaluations do not share resolved flags.

Written by the indexing model from the issue text.