Relationship validation/schema is not IMDF compliant

[jillesvangurp]

Summary

The in-app IMDF validator rejects relationship features that follow the current IMDF relationship model.

Evidence

• src/lib/imdf/validate.ts:115 hard-requires relationship geometry to be null.
• src/lib/imdf/validate.ts:192 validates a required references array for relationship features.
• src/lib/imdf/featureCatalog.ts:209 defines relationship required fields as category, numeric direction, and references.

Why this is a bug

Current IMDF relationship features are modeled with origin, intermediary, and destination references, and geometry may be null or present as GeoJSON. The current validator/schema rejects those features.

Reference: https://docs.ogc.org/cs/20-094/relationship/index.html

Reproduction

1. Create/import a relationship feature that uses origin + destination references (no references array), with direction as "directed" or "undirected".
2. Run through validateImdfDatasetFiles (directly or via archive import).
3. Observe errors for missing properties.references and/or geometry requirements.

Expected

• Accept IMDF relationship fields (origin, optional intermediary, destination, string direction enum).
• Allow relationship geometry to be null or a valid GeoJSON geometry.

Impact

Interoperability with spec-conformant IMDF datasets is broken; valid relationship data is rejected.