Fix HtmlToDjot remaining serialization edge cases

[dereuromark]

Summary

• keep block-content table cells valid by degrading them to single-line cell text instead of emitting malformed Djot tables
• preserve multiline footnote definitions through cleanup so later paragraphs stay inside the footnote body
• escape problematic link labels and fall back to raw inline HTML for images that cannot be represented safely in Djot labels
• raw-fall back the entire anchor when a linked image would otherwise be broken by label escaping

Testing

• vendor/bin/phpunit tests/TestCase/Converter/HtmlToDjotTest.php
• vendor/bin/phpunit tests/TestCase/Converter/HtmlToDjotTest.php tests/TestCase/RoundTripTest.php tests/TestCase/DjotConverterTest.php tests/TestCase/ProfileTest.php
• vendor/bin/phpstan analyse src --no-progress
• composer cs-check
• vendor/bin/phpunit

[codecov]

Codecov Report

❌ Patch coverage is 98.11321% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 93.68%. Comparing base (59a5e5f) to head (00a91c0).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/Converter/HtmlToDjot.php	98.11%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #174      +/-   ##
============================================
+ Coverage     93.66%   93.68%   +0.02%     
- Complexity     3201     3220      +19     
============================================
  Files            93       93              
  Lines          8125     8171      +46     
============================================
+ Hits           7610     7655      +45     
- Misses          515      516       +1     

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