ocda.db: read a document language of a database

A database now holds every language of its document, so reading one
means naming a language. Three small pieces:

- spineDbLanguages says what languages a file holds. Its own template
  rather than part of the reader (asking a file what it holds does not
  instantiate the object setter and the markup regexes with it).

- abstractionLoad and spineDocFromArtefact take a language and pass it
  to dbReadFile, which resolves it to a doc_id, takes that language
  document where there is one, and lists the languages and stops where
  there is a choice.

- spineArtefactLanguages turns an artefact into the documents to build:
  one for a .ssp, and for a database the languages it holds filtered by
  --lang. The artefact loop then has a language loop inside it, so a
  single .ocda.db argument builds every language, as the pod it came
  from would.

--db-round-trip takes --lang for the same reason, and  ocda-db looses
--parallel (would cause a race).

(assisted by Claude-Code)

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