artifact: testing/deterministic/src/test/kotlin/RouterReducerTest.kt
purpose: Test DefaultRouterReducer — verify state transitions for SteeringNoteAddedEvent and each workflow lifecycle event.
depends_on: task-009, task-019
non_goals: Testing projector, context builder, or facade logic
done_when: All reducer state transition tests pass with ./gradlew :testing:deterministic:test --tests RouterReducerTest