More robust JSON-LD processing

Only specific forms of compacted JSON-LD are accepted for now, and this might lead to errors or unexpected results (for example, if rdflib's behaviour changes)

Start by locating the JSON-LD processing path and reviewing how compacted JSON-LD is currently accepted. Reproduce the error or unexpected-result cases described in the issue, then define coverage for additional compacted forms and verify behavior remains robust if rdflib changes.

Written by the indexing model from the issue text.