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 SDK getFlags into the engine and inspect how segments and per-evaluation context are passed. Use the engine-test-data tag containing the flag dependency cases as the first validation point; done means $.flags conditions work for identity-less evaluation, environments without them avoid added per-evaluation cost, and concurrent evaluations do not share resolved flags.

索引モデルが issue の本文から書いたものです。