Preserve unsupported figures as raw HTML blocks

[dereuromark]

Summary

• fall back to raw HTML block syntax when a <figure> cannot be represented losslessly in Djot
• keep the structured Djot path for supported image and blockquote figures
• add regression coverage for unsupported figure bodies and figure-level attributes

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
• vendor/bin/phpstan analyse src --no-progress
• vendor/bin/phpunit

[codecov]

Codecov Report

❌ Patch coverage is 84.61538% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.95%. Comparing base (6b165b4) to head (72c0db7).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/Converter/HtmlToDjot.php	84.61%	4 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #171      +/-   ##
============================================
- Coverage     93.98%   93.95%   -0.04%     
- Complexity     3139     3152      +13     
============================================
  Files            93       93              
  Lines          8017     8041      +24     
============================================
+ Hits           7535     7555      +20     
- Misses          482      486       +4     

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