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 integration, then inspect the engine-test-data dependency and its flag dependency cases. Verify that $.flags conditions work for identity-less evaluations, environments without such conditions avoid per-evaluation cost, and resolved flags remain isolated across concurrent evaluations.

索引モデルが issue の本文から書いたものです。