Add InlineFootnotesExtension for inline footnotes

[dereuromark]

Summary

• Add InlineFootnotesExtension that converts [content]{.fn} spans to inline footnotes
• Add registerInlineFootnote() method to HtmlRenderer for extension use
• Inline footnotes integrate seamlessly with regular footnotes (shared numbering)
• Supports full inline formatting in footnote content
• Comprehensive test coverage (13 tests)

Usage

$converter = new DjotConverter();
$converter->addExtension(new InlineFootnotesExtension());

$html = $converter->convert('Text[An inline footnote]{.fn} here.');

Why This Syntax?

This follows the approach discussed in jgm/djot#286. Pandoc's ^[...] syntax conflicts with djot's superscript ^text^, so the span-with-class approach provides inline footnotes without parser changes.

Edge Cases Tested

• Multiple inline footnotes
• Mixed inline + regular footnotes
• Formatting inside footnotes
• Empty footnotes
• Multiple classes including .fn
• Numbering reset between renders
• Backlink generation

[codecov]

Codecov Report

❌ Patch coverage is 94.73684% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.51%. Comparing base (df215b7) to head (54b095d).

Files with missing lines	Patch %	Lines
src/Extension/InlineFootnotesExtension.php	92.30%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##             master      #90   +/-   ##
=========================================
  Coverage     93.50%   93.51%           
- Complexity     2302     2313   +11     
=========================================
  Files            78       79    +1     
  Lines          6070     6107   +37     
=========================================
+ Hits           5676     5711   +35     
- Misses          394      396    +2     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.