artifact: core/router/src/main/kotlin/com/correx/core/router/RouterProjector.kt
purpose: Implement RouterProjector as Projection<RouterState> backed by DefaultRouterReducer, providing initial() and apply() methods per the core architecture pattern.
depends_on: task-009
non_goals: Implementing repository, context builder, or facade logic
done_when: RouterProjector produces correct RouterState from an empty event list and from a sequence of events