Review finding: `add_users_to_group_at`, `set_group_status_at` and
`remove_members_from_group_at` handed the replicated source stamp to the
cache as its publication time. `LockedCache::exec` drops a publication whose
time is behind the entity's load time, so a group edit whose source time
predated this node's startup was persisted but never reached the cache, and
the receiver's following status write failed with `NoSuchGroup`.
The stamp now stays on `GroupInfo::update_at` only; the group entity and the
membership index are published with the local clock. The regression drives
`IamSys` through a startup-shaped cache load and then add, status and removal
with hour-old source stamps, asserting each is readable at once.
唐小鸭
cc68d18783