New features:
- TripleQuery fluent pattern-matching builder
- merge/diff/intersection/subgraph graph operations
- HeteroGraph O(1) adjacency index, degree methods, to_knowledge_graph()
- HyperGraph KG interop and query methods
- PPR and label propagation wrappers (graphops delegation)
- Predicate index for O(d) triples_with_relation()
- oxttl/oxrdf migration from deprecated rio parsers
- Format round-trip property tests