Function plan_cache_cost 

pub fn plan_cache_cost(steps: &[CompactionStep]) -> Option<usize>

The prompt-cache-invalidation index of a whole plan = the earliest break across its steps (an earlier break invalidates everything after it, so the minimum dominates the cost). None means the plan is entirely prefix-safe and preserves the prompt cache — the cache-aware planner’s goal whenever the safe steps can free enough.