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 getFlags and the engine-test-data dependency or test setup. Trace how segments reach the engine, then run the dependency cases and add coverage for no per-evaluation cost, evaluation scoping, and identity-less $.flags and $.environment resolution; done means all acceptance criteria pass.

Written by the indexing model from the issue text.