fix: replace UseAdvancedExtensions with selective pipeline to fix phantom table columns (#68)

[forest6511]

Summary

• Fixes Workaround: Markdig subscript extension (~) breaks table cell parsing with bold (**) #68: Bold markers (**) combined with tilde (~) inside table cells caused Markdig to create phantom columns
• Root cause: UseAdvancedExtensions() enables the subscript extension where ~ is a delimiter. When **~$1.50** appears in a table cell, the subscript parser interferes with cell boundary detection, inflating column count
• Fix: Replace UseAdvancedExtensions() with explicit extension registration, using UseEmphasisExtras(EmphasisExtraOptions.Strikethrough) to keep ~~strikethrough~~ support while disabling subscript (~) and superscript (^)

Test plan

• 302 tests passing (297 existing + 5 new regression tests)
• Verified: bold-only tables parse correctly (3 columns)
• Verified: tilde-only tables parse correctly (3 columns)
• Verified: bold+tilde combination now parses correctly (3 columns, was 4)
• Verified: ~~strikethrough~~ still works in tables
• Verified: literal ~ preserved in cell text

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.69%. Comparing base (e6e81ce) to head (5873c57).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #69      +/-   ##
==========================================
+ Coverage   88.54%   88.69%   +0.15%     
==========================================
  Files          27       27              
  Lines        1405     1424      +19     
  Branches      179      179              
==========================================
+ Hits         1244     1263      +19     
  Misses        122      122              
  Partials       39       39              

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