Build CQRS read-model projection tests - write-model + read-model consistency tests, projection-replay determinism, projection-versioning + zero-downtime swap, eventual-consistency-window assertions. Per martinfowler.com CQRS reference. Use when a read model is derived from a write-model event stream - adding a projection, migrating a projection schema, or chasing a "I changed it but the UI shows the old value" report.