Support `$.flags` context values in segment conditions

[khvn26]

Support $.flags.<feature name>.<enabled|value|variant> 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.