- Immediately after access check, return 403 if patch.isShared !== undefined
and !access.isOwner — blocks sharees from mutating list_shares
- Guard message: 'Only the list owner can change sharing settings'
- Sharees may still PATCH { name } (rename test stays green)
- Update stale comment: 'Reconcile list_shares on visibility change (owner only)'
- Closes T-04-08 (elevation of privilege) and T-04-05 (shared root cause)