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, as are the first known GivenShare.resourceType / GivenShare.resourceName.