Add line block and raw inline round-trip support

[dereuromark]

Summary

Adds round-trip support for:

1. Line blocks (| Line syntax) - detects div.line-block and converts back to pipe-prefixed lines
2. Raw inline (`content`{=format} syntax) - wraps in span with data-djot-raw attribute

Line blocks

• Add processLineBlock() to detect and convert div.line-block elements
• Add extractLineBlockLines() to parse <br>-separated content
• Add 3 round-trip tests

Raw inline

• Modify HtmlRenderer.renderRawInline() to wrap content in span with data-djot-raw
• Add processRawInline() and getInnerHtml() to HtmlToDjot
• Add 3 round-trip tests (HTML, complex HTML, non-HTML format)
• Update Round-Trip Limitations documentation

This is a follow-up to #157 which added extended round-trip support for other elements.

[codecov]

Codecov Report

❌ Patch coverage is 95.71429% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.78%. Comparing base (51a4c33) to head (c2e081f).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/Converter/HtmlToDjot.php	95.16%	3 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #158      +/-   ##
============================================
+ Coverage     93.74%   93.78%   +0.04%     
- Complexity     2980     3007      +27     
============================================
  Files            92       92              
  Lines          7691     7758      +67     
============================================
+ Hits           7210     7276      +66     
- Misses          481      482       +1     

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