Find Feature Activation Order

You are building a feature-flag system for a product. There are `n` features labeled `0` to `n - 1`. Each dependency pair `[a, b]` means feature `a` depends on feature `b`, so `b` must be active before `a` can be activated. You are also given: - a set `active` of features that are already enabled, and - a target feature `t` that a user wants to activate. A feature can be activated only after **all** of its direct and indirect prerequisites are active. If a prerequisite is not active yet, it must be activated first. Write a function that returns **one valid order** of newly activated features needed to enable `t`. Rules: 1. Only include features that are not already active. 2. The returned order must activate prerequisites before any dependent feature. 3. If `t` is already active, return an empty list. 4. If `t` cannot be activated because its dependency chain contains a cycle, return `null`. Example: - `dependencies = [[3,1],[3,2],[2,0]]` - `active = {1}` - `t = 3` A valid answer is `[0,2,3]`. Implement this efficiently.