Support merging user-provided JSON-LD frame with built-in default

[ddeboer]

Problem

When using docgen from-shacl -f frame.jsonld, the user must provide a complete JSON-LD frame that duplicates all of docgen's built-in defaults (e.g. targetClass, datatype, path, severity type coercions) plus their own custom properties.

This means:

• If docgen updates its default frame, consumers' custom frames silently become out of sync.
• Users must understand docgen's internal framing requirements just to add a single custom property.

For example, in dataset-register we only need to add two custom properties (nde:futureChange, nde:version) but have to maintain a full copy of the default frame to do so: shacl.frame.jsonld.

Proposal

Have docgen deep-merge a user-supplied frame with its built-in default frame before calling jsonld.frame(). This way the -f flag provides additions to the default rather than a full replacement.

A user's frame would then only need the custom parts:

{
  "@context": {
    "nde": "https://def.nde.nl#",
    "nde:futureChange": {},
    "nde:version": {}
  }
}

Since JSON-LD framing only accepts a single frame object, the composition must happen at the application level — which makes docgen the natural place for it.