Match Fumadocs TOC and list behavior

[timkpaine]

Description

Match Fumadocs behavior by highlighting every heading visible in the viewport, keeping the TOC rail aligned with the active range, and removing extra paragraph spacing from Sphinx tight lists.

Type of Change

• Bug fix
• New feature
• Documentation update
• Refactor / code cleanup
• CI / build configuration
• Other (describe below)

Checklist

• Linting passes (make lint)
• Tests pass (make test)
• New tests added for new functionality
• Documentation updated (not applicable)
• Changelog / version bump (not applicable)

[github-actions]

Test Results

44 tests  +3   44 ✅ +3   3s ⏱️ -1s
 1 suites ±0    0 💤 ±0 
 1 files   ±0    0 ❌ ±0 

Results for commit 29c0ed1. ± Comparison against base commit 8376c22.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.97%. Comparing base (8376c22) to head (29c0ed1).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main       #8      +/-   ##
==========================================
+ Coverage   91.75%   91.97%   +0.21%     
==========================================
  Files           6        6              
  Lines         449      461      +12     
  Branches       61       61              
==========================================
+ Hits          412      424      +12     
  Misses         16       16              
  Partials       21       21              

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