Fix extension render idempotence

[dereuromark]

Summary

• reset per-document extension state before rendering so TOC data does not leak across repeated conversions
• make repeated rendering of the same parsed document idempotent for heading permalinks and class-adding extensions
• add regression coverage for repeated convert() and render() calls and document the extension lifecycle behavior

[codecov]

Codecov Report

❌ Patch coverage is 92.30769% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.83%. Comparing base (9163397) to head (2f1b679).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/Extension/HeadingPermalinksExtension.php	92.30%	1 Missing ⚠️
src/Node/Node.php	90.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #119      +/-   ##
============================================
- Coverage     93.84%   93.83%   -0.02%     
- Complexity     2349     2363      +14     
============================================
  Files            79       79              
  Lines          6224     6245      +21     
============================================
+ Hits           5841     5860      +19     
- Misses          383      385       +2     

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