Metric validation — correlation to outcomes and sensitivity to change

This is Lesson 4.3: Cost-Aware Evaluation. In the previous lesson, metrics were classified as blocking versus optimization and mapped to measurement archetypes. Now the lesson addresses a problem that hits every team running LLM-based evaluations: the evaluation suite costs more than running the system itself. That is not sustainable. By the end of this lesson, the deliverable is a Cost Allocation Plan that cuts evaluation costs by 90% without losing the ability to make ship decisions. Budget is allocated by segment risk, evaluations are routed through tiered judge cascades, and 100% coverage is preserved for safety-critical failures. This is portfolio-ready work that can be presented to leadership with the message: "Here is how we stay within budget without flying blind on quality."