Revisit: dedicated 'CODECHECK certificate ID' property on Wikidata instead of P528 catalog code

Certificates are identified on Wikidata by `P528` *catalog code* carrying the certificate ID, qualified with `P972` *catalog* → [Q141254857](https://www.wikidata.org/wiki/Q141254857), the register. This was always meant as the interim form (see the model discussion in #50): a catalog code says nothing without naming its catalog, and the model validator rejects a `P528` statement that lacks the qualifier, so it is correct and queryable — it is just generic.

The permanent form is a dedicated external identifier property, "CODECHECK certificate ID", with a formatter URL of `https://codecheck.org.uk/register/certs/$1/`. That would make a certificate ID resolve from Wikidata the way a DOI or an ORCID does, and make `haswbstatement` lookups on it exact.

**The interim solution is accepted for now.** This issue is to revisit it, not to block anything.

What it would involve:

- A [property proposal](https://www.wikidata.org/wiki/Wikidata:Property_proposal) under *Authority control*, which takes weeks and needs the usual evidence: the identifier is stable, externally maintained, and used by more than one item. The register (130+ certificates, publicly listed, each with an archived DOI) is a reasonable case.
- Once created, migrating the existing certificate items from `P528`+`P972` to the new property. That is a QuickStatements batch of its own, generated from the same model.

Worth deciding **before** the certificates batch is pasted if the proposal is going to happen soon, since migrating 130+ items is more work than writing them right the first time. If not, the interim form stands and this can be picked up whenever.

Blocked on nothing; depends on #50.

Review #50 and the Wikidata property proposal guidance first; use the CODECHECK register and its 130+ certificates as evidence for identifier stability and adoption. If the proposal succeeds, generate the QuickStatements migration from the existing model and verify that each certificate moves from P528/P972 to the dedicated property.

Written by the indexing model from the issue text.