Fix duplicate IDs for multiple footnote references

[dereuromark]

Summary

When the same footnote is referenced multiple times in a document, the renderer now generates:

• Unique HTML-compliant IDs with suffixes: fnref1, fnref1-2, fnref1-3
• Multiple backlinks with numbered superscripts: ↩︎¹ ↩︎² ↩︎³

Single references keep the simple format (no suffix, no superscript on backlink).

Example

Input:

First ref[^note].
Second ref[^note].
Third ref[^note].

[^note]: The footnote content.

Before (invalid HTML - duplicate IDs):

<a id="fnref1" ...>1</a>
<a id="fnref1" ...>1</a>
<a id="fnref1" ...>1</a>
...
<a href="#fnref1">↩︎</a>

After (valid HTML):

<a id="fnref1" ...>1</a>
<a id="fnref1-2" ...>1</a>
<a id="fnref1-3" ...>1</a>
...
<a href="#fnref1">↩︎¹</a> <a href="#fnref1-2">↩︎²</a> <a href="#fnref1-3">↩︎³</a>

Test plan

• Added unit tests for multiple and single footnote references
• Updated official test suite expected output
• All 787 tests pass

Related: jgm/djot#348

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.19%. Comparing base (aa2c64d) to head (895e467).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master       #6      +/-   ##
============================================
+ Coverage     92.14%   92.19%   +0.04%     
- Complexity     1392     1399       +7     
============================================
  Files            53       53              
  Lines          3578     3598      +20     
============================================
+ Hits           3297     3317      +20     
  Misses          281      281              

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