fix: no endnotes section for unreferenced footnotes

[dereuromark]

What

The endnotes section was emitted whenever any footnote definition was collected (collectedFootnotes), regardless of whether it was referenced. A footnote defined but never referenced produced a stray, empty section:

Before, text\n\n[^f]: note:

<p>text</p>
<section role="doc-endnotes">
<hr>
<ol>
</ol>
</section>

Fix

Gate the section on referenced footnotes only (footnoteNumbers, populated when a reference is rendered). An unreferenced definition now produces no output. Referenced footnotes are unchanged.

Backport of the footnote part of markup-carve/carve-php#31 (shared parser lineage).

Tests

Added a case asserting an unreferenced footnote definition emits no doc-endnotes section. Full suite and official corpus green.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.89%. Comparing base (32c0d50) to head (f434bf1).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master     #236   +/-   ##
=========================================
  Coverage     91.89%   91.89%           
+ Complexity     3523     3522    -1     
=========================================
  Files           106      106           
  Lines          9958     9958           
=========================================
  Hits           9151     9151           
  Misses          807      807           

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