Imported ontologies with rdfs:subClassOf hierarchies have no relationships (Brick: 1,530 entities, 0 edges)

[mathewtbenjamin]

Describe the bug

After #96, the Brick ontology imports successfully — but the resulting graph has 1,530 entity types and 0 relationships. Every node is disconnected, which makes the imported ontology unusable for exploration.

Repro

1. Download Brick 1.4 as RDF/XML (content negotiation on https://brickschema.org/schema/Brick, Accept: application/rdf+xml)
2. Import it (with fix: support rdf:Description typed-node syntax in RDF import (Brick) #96 applied so the import succeeds)
3. Observe: 1,530 entities, 0 relationships

Root cause

The relationship mapper only builds relationships from owl:ObjectProperty elements carrying rdfs:domain/rdfs:range pairs. Brick 1.4 contains just 2 rdfs:domain and 5 rdfs:range statements in the whole 5.2 MB file — its actual graph structure is the class taxonomy:

• 2,016 rdfs:subClassOf statements between classes (e.g. brick:DDAHU rdfs:subClassOf brick:Air_Handling_Unit), which the parser ignores entirely.

This affects any ontology whose structure is primarily a class hierarchy (most published OWL taxonomies), not just Brick.

Expected behavior

rdfs:subClassOf references between imported classes should be extracted as relationships so the taxonomy is visible and navigable in the graph.

(Brick also expresses property domain/range constraints via SHACL property shapes — sh:property → sh:path/sh:class — which could be a further enhancement, but the subClassOf hierarchy is the dominant structure and the highest-value fix.)

Happy to submit a PR for this — follow-up to #96 as noted in its description.