collapseByReceiver

Collapses a flat list of GivenShare — which may carry several rows for one (receiver, resourceUri) when a grant writes multiple implied acl:modes (e.g. Read+Append for "Add", Read+Write for "Edit") — into a single row per pair carrying the strongest ShareMode. Each receiver therefore appears once per resource at its effective access level. The earliest known GivenShare.createdAt is preserved.