Add abbreviation and escaped character round-trip support

[dereuromark]

Summary

• Adds EscapedText node class to preserve escape sequences (\*, \_, etc.) during round-trips
• Stores abbreviation definitions on Document and outputs them via <template data-djot-abbreviations> element
• HtmlToDjot extracts abbreviations from template element and escaped chars from data-djot-escaped spans
• Removes both from the "Round-Trip Limitations" section in documentation

Changes

• src/Node/Inline/EscapedText.php - New node class for escaped punctuation
• src/Node/Document.php - Added abbreviations property with getter/setter
• src/NodeType.php - Added ESCAPED_TEXT constant
• src/Parser/BlockParser.php - Store abbreviations on document after parsing
• src/Parser/InlineParser.php - Create EscapedText nodes instead of text for escaped chars
• src/Renderer/HtmlRenderer.php - Render abbreviation template and escaped text spans
• src/Converter/HtmlToDjot.php - Extract abbreviations and escaped characters
• tests/TestCase/RoundTripTest.php - Added 8 new tests
• tests/TestCase/Parser/InlineParserTest.php - Updated to expect EscapedText nodes
• docs/guide/converters.md - Updated round-trip documentation

[codecov]

Codecov Report

❌ Patch coverage is 96.15385% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.80%. Comparing base (60e5532) to head (3157dd4).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/Node/Inline/EscapedText.php	71.42%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #159      +/-   ##
============================================
+ Coverage     93.78%   93.80%   +0.01%     
- Complexity     3007     3027      +20     
============================================
  Files            92       93       +1     
  Lines          7758     7809      +51     
============================================
+ Hits           7276     7325      +49     
- Misses          482      484       +2     

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