POST /v1/workspaces/{id}/blobs/{hash} - associate another workspace owned by the same Profile with one content-addressed blob.
A selection moved across workspaces carries blob references in its CRDT cutting, and the target must be associated before those references are published or it holds a reference it may not read.
Named for the association rather than a copy, because no bytes move: one stored object serves every workspace associated with it, and the association is the permission. The old name described a layout that filed one picture once per workspace, and a caller that still believes it would duplicate a picture library across workspaces.