Footnote Inline Void Fix

Goal

Make footnote references behave like real inline atoms instead of editable text.

Phases

• Load prior learnings and inspect the current footnote runtime/tests
• Add red tests for inline-void ref behavior and backlink navigation
• Implement the footnote reference inline-void model and renderer/nav fixes
• Verify with targeted tests, package checks, registry build, lint, and browser-use

Notes

• The current spec now says footnote refs should be inline void atoms.
• Runtime is still wrong: footnoteReference is inline but not void.
• Browser symptom to kill:
  • Backspace after the ref edits the visible identifier.
  • Backlink navigation triggers generic chrome instead of clean navigation.
• Result:
  • footnoteReference is now an inline void atom with an empty child sentinel
  • backlink focus lands on the nearest stable sibling text point instead of a node-range selection
  • browser proof on /docs/footnote showed backlink jump landing on the . after [1], only one toolbar on screen, and one Backspace removing the whole ref instead of the digit