Chapter 8 — Observe Pipeline Failures Without Turning UI Feedback into Control

Chapter 8 is a lab, not a fresh application. Start with the completed Chapter 7 filters-and-reducers checkpoint, then deliberately make one pipeline filter fail. The exercise shows how a FeatureCell™ can expose a finalized feature error while the UI remains an observer rather than a second source of pipeline authority. You will keep the Chapter 7 CRUD workflow, pure filter, and ordered reducers. The lab adds one intentional throwing filter, an errors() callback, and a global error response.