artifact: core/router/src/main/kotlin/com/correx/core/router/RouterRepository.kt
purpose: Implement DefaultRouterRepository using EventReplayer<RouterState> to rebuild RouterState from the event store for a given session.
depends_on: task-010
non_goals: Implementing context builder, facade logic, or infrastructure wiring
done_when: DefaultRouterRepository.getRouterState(sessionId) delegates to EventReplayer.rebuild(sessionId) and returns the projected state
