artifact: core/router/src/main/kotlin/com/correx/core/router/RouterReducer.kt
purpose: Implement RouterReducer — reduces RouterState from RouterState events, specifically SteeringNoteAddedEvent and workflow lifecycle events (started, completed, failed, paused, resumed, stage completed, stage failed).
depends_on: task-006
non_goals: Implementing projector, repository, context builder, or facade logic
done_when: DefaultRouterReducer.reduce() correctly updates RouterState fields for every known event type and returns unchanged state for unknown events