Add explicit AST transforms for heading shifts and inline footnotes

[dereuromark]

Summary

• add an explicit AST transform pipeline to DjotConverter
• introduce HeadingLevelShiftTransform and InlineFootnotesToParenthesesTransform
• keep InlineFootnotesExtension HTML-only and move non-HTML fallback behavior out of beforeRender() mutation
• document explicit transform usage in the API and extension docs

Why this variant

• keeps rendering pure: parse -> transform -> render
• avoids hidden AST mutation during render()
• avoids renderer-specific extension APIs and shared render-time option leakage
• makes ownership explicit by returning transformed document copies

Prototype tradeoff

• slower for one-shot parse+transform+render flows because the transform clones once up front
• better suited to parse-once/render-many flows because rendering stays simple after the one-time transform

Verification

• php vendor/bin/phpunit --colors=never
• composer stan
• composer cs-check

[codecov]

Codecov Report

❌ Patch coverage is 97.36842% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 93.65%. Comparing base (0947440) to head (5bc7cfd).
⚠️ Report is 5 commits behind head on master.

Files with missing lines	Patch %	Lines
src/Transform/HeadingLevelShiftTransform.php	92.30%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #149      +/-   ##
============================================
- Coverage     93.69%   93.65%   -0.05%     
- Complexity     2642     2649       +7     
============================================
  Files            90       92       +2     
  Lines          6983     7002      +19     
============================================
+ Hits           6543     6558      +15     
- Misses          440      444       +4     

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