Add HTML→Djot enhancements: attribution, wrapper unwrap, MathML

[dereuromark]

Summary

Enhances the HTML→Djot converter with three new features inspired by Pandoc's capabilities:

• Blockquote attribution: Extracts <footer> or <cite> elements from blockquotes and converts them to Djot's quoted attribution format (adds a blank > line followed by the attribution text)
• Wrapper div unwrapping: When a div with attributes contains a single block child, applies the attributes directly to the child instead of creating a fenced div
• MathML conversion: Converts <math> elements to Djot math syntax by extracting LaTeX from alttext attribute, <annotation> element with tex/latex encoding, or text content fallback

All features include comprehensive tests.

[codecov]

Codecov Report

❌ Patch coverage is 98.30508% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 93.94%. Comparing base (c7941ca) to head (54ba8f1).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/Converter/HtmlToDjot.php	98.30%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #165      +/-   ##
============================================
+ Coverage     93.91%   93.94%   +0.03%     
- Complexity     3075     3106      +31     
============================================
  Files            93       93              
  Lines          7891     7950      +59     
============================================
+ Hits           7411     7469      +58     
- Misses          480      481       +1     

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