CVE-2026-75837: Grav: Missing admin.super guard on core group blueprint access field allows admin.users operator to escalate to super-admin

The core Flex group blueprint system/blueprints/user/group.yaml (access field, lines 48-55) omits the security@: admin.super field guard that its sibling account blueprint carries (account.yaml:131/138/150, added by the CVE-2026-42613 fix). A delegated non-super operator holding admin.users.update can therefore save a group whose access map contains admin.super: true, which UserGroupObject::authorize then grants to every member of that group, a full privilege escalation to super-admin (scheduler/cron RCE, Twig eval). This is a distinct file, sink, and fix from all four related advisories.