Add citation syntax with locator + bibliography key

[swernerx]

Summary

Investigate support for citation syntax aimed at presentation or handout workflows. iA Presenter supports locator-plus-key references such as [p. 23][#Doe:2006].

Why

• This is distinct from ordinary links and footnotes.
• It could unlock a useful handout/export story, but it likely needs metadata and rendering policy work beyond the core parser.

Scope

• Decide whether citation parsing belongs in core ferromark, presentation mode only, or a higher-level preprocessor.
• Specify accepted key grammar and locator grammar.
• Decide output behavior: plain link, semantic citation span, footnote, bibliography collection, or deferred metadata only.

Acceptance Criteria

• A minimal citation grammar is written down.
• Output expectations are defined for HTML and presentation use.
• Non-goals are explicit if bibliography formatting is deferred.

References

• iA Markdown Guide
  https://ia.net/presenter/support/basics/markdown
• Pandoc citation syntax is a useful contrast point even though it uses a different form (@key).
  https://pandoc.org/MANUAL.html#citation-syntax