Design a feature flag system

Design a feature flag system for [Application] that supports the following capabilities: simple boolean on/off flags, percentage-based gradual rollouts, user-segment targeting using user ID or attribute values, and kill switches for emergency feature disablement. Design the flag definition schema including name, type, targeting rules, and default state. Describe the server-side evaluation API and the client SDK interface. Explain the flag lifecycle from creation through graduated rollout to full release and final cleanup, and how to avoid flag debt accumulating over time.