Refactor HtmlRenderer state into explicit render context

[dereuromark]

Summary

• move HtmlRenderer mutable per-render state into an explicit RenderContext
• add fragment/document fragment rendering paths that reuse the active context instead of resetting renderer state
• add renderer tests covering fragment rendering with heading and footnote state

Testing

• php vendor/bin/phpunit --colors=never

[codecov]

Codecov Report

❌ Patch coverage is 98.38710% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 93.64%. Comparing base (741e740) to head (1cddc75).
⚠️ Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
src/Renderer/HtmlRenderer.php	98.11%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #145      +/-   ##
============================================
- Coverage     93.70%   93.64%   -0.07%     
- Complexity     2653     2659       +6     
============================================
  Files            88       89       +1     
  Lines          6977     7003      +26     
============================================
+ Hits           6538     6558      +20     
- Misses          439      445       +6     

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