Shared plans are saved synced or as a copy; enrollments pin a version

Plans and workouts are versioned and immutable per version. A shared item saved to a library is either synced (follows the author's latest version; editing makes a personal copy) or a copy (forked from one version). An enrollment pins one plan version, and a synced update applies only from the next week, so a plan never changes under someone mid-week and every session keeps the version it was done against. Coaching reuses this: an assigned plan is a synced enrollment the athlete can skip, move or log but not edit.