Promote the SI defining constants to named quantity nodes

[jviotti]

Signed-off-by: Juan Cruz Viotti jv@jviotti.com

[augmentcode]

🤖 Augment PR Summary

Summary: This PR updates the SI defining-constant schemas so that, when converted to JSON-LD/RDF, they become named qudt:Quantity nodes rather than bare numeric literals.

Changes:

• Annotated the 2019 SI defining constants (Avogadro, Boltzmann, caesium hyperfine frequency, elementary charge, speed of light, luminous efficacy, Planck) with x-jsonld-self, x-jsonld-type, x-jsonld-value, and unit x-jsonld-constants.
• Added a new “Calibration Certificate” example schema that references the caesium defining constant and a prefixed SI unit (millihertz) for a measured offset.
• Added a corresponding example test that validates both JSON Schema validity and the expected RDF shape (constant becomes a named quantity node).
• Extended each defining-constant schema test to assert the new RDF output, including the minted constant IRI and associated unit/value predicates.

Technical Notes: The approach relies on JSON Schema extension keywords to drive JSON-LD emission, adding stable IDs for constants while keeping the instance data format as simple numeric values.

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[augmentcode]

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[augmentcode]

examples/calibration-certificate.json:48 The measuredOffset field references the millihertz quantity schema, which currently has exclusiveMinimum: 0, so a zero or negative frequency offset would be rejected. If measuredOffset is meant to represent a signed deviation from the defining constant, consider using a schema that permits 0 and negative values.

Severity: medium

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