Preview footnote definitions from references

[Azganoth]

Summary

Footnote references currently expose their editable [^label] source but provide no way to inspect the corresponding definition without finding it elsewhere in the document.

Allow a footnote reference to reveal an accessible, bounded preview of its resolved definition and provide an explicit way to navigate to that definition. Missing definitions should produce a clear missing-definition state rather than an empty or broken preview.

This issue covers preview and navigation only. It does not change the established rule that directly editing a reference label does not rename or modify definitions.

Expected behavior

• A reference with a matching definition can reveal a delayed preview through pointer interaction.
• The same preview is available through a keyboard-accessible interaction; it is not hover-only.
• The preview presents a bounded, non-editable representation of the definition content and truncates its presentation when necessary without modifying the document.
• A reference without a matching definition presents an explicit missing-definition state.
• The user can navigate from the reference or preview to the matching definition, moving editor focus and selection to that definition.
• Preview and navigation behavior do not prevent the reference's existing editable source-projection interaction.
• Multiple references with the same label resolve to the same definition.
• Showing, hiding, or navigating from the preview does not modify Markdown, dirty state, or undo history.
• Directly editing [^label] continues to affect only the edited reference and does not silently rename related references or definitions.

Related context

• Related issues or pull requests: Project footnote-reference source in the editor #60
• Documentation and established behavior:
  • docs/specification.md — Footnote definition presentation
  • docs/specification.md — Footnote-reference source projection

Done when

• Reference labels resolve against footnote definitions in the active ProseMirror document.
• Existing definitions produce a bounded preview and unresolved labels produce an explicit missing-definition state.
• The preview is available through both pointer and keyboard interaction.
• An explicit navigation action focuses the matching definition without replacing the existing reference-editing gesture.
• Opening, closing, and navigating from a preview preserve document content, dirty state, undo history, and source-projection behavior.
• Focused tests cover resolved and missing definitions, repeated references, pointer and keyboard access, navigation, and source-projection compatibility.
• The specification and Unreleased changelog describe the accepted interaction.

Notes

Interaction decision before Ready

A normal interaction with a footnote reference currently activates its editable source projection. The preview and navigation interaction must not take that gesture away.

Before moving this issue to Ready, decide:

• which gesture performs navigation;
• whether keyboard and pointer users reach the same control;
• whether the preview contains an interactive navigation action.

If the preview contains a focusable navigation action, it should use popover semantics rather than presenting interactive content inside a semantic tooltip.

Out of scope

• Renaming a definition and all of its references.
• Inserting new references or definitions.
• Redesigning the footnote-definition block.
• Changing footnote serialization or source-projection behavior.