Add literature references (BibTeX-style key + pages) to model elements

[goloroden]

What is this feature about?

Model elements should be able to cite external literature precisely and machine-readably, not just in prose. When a term, aggregate, command, or read model rests on a specific source, the model should carry an exact reference to it. The difference matters for humans and AI alike: "Göhner's frame of reference" versus "Göhner (1987, Bewegungsanalyse im Sport, p. 7)" leads to very different results.

The proposed shape is BibTeX-oriented, similar to pandoc citations:

references:
  - file: path/to/literature.bib
    key: "@goehner87"
    pages: 7

The reference should be attachable wherever it is useful – on ubiquitous-language terms, aggregates, read models, commands, and so on. Today ESDM has no such concept.

What needs to be done to implement this feature?

• Design a reusable references building block (a .bib file plus a citation key and optional pages), following pandoc's citation model as a guide.
• Decide which element kinds can carry references and how the field attaches to them.
• Decide how references are validated (for example that the key exists in the given .bib file) and whether that is a linter concern.
• Document the feature and how references surface in the outputs (view, glossary, and the future documentation command).

What else should we know?

Open design questions (user-facing, need sign-off):

• The exact field shape and where it is allowed.
• Whether the .bib file is resolved and validated by the tooling or treated as an opaque pointer.

Further notes:

• Origin: raised by a user modeling a sports-science domain, where terminology rests on specific academic sources.
• Related to Provide a stable reference notation for model elements so external documents can link to them #8 (inbound element references); also relates to the multilingual dictionary work (Decide whether and how ESDM should support multilingual terminology (a dictionary) #10), where references are a component of each term's translation.