refactored index

[daveshanley]

no breaking changes or changes to the public contract, but a significant clean up, refactor, breakdown, and tune-up.

The index is where the library was born, inside vacuum in 2022. It's been abused, hacked, tuned, broken, fixed, grown, changed, and hammered for years. It's not easy to navigate or understand.

This is a stage one refactor to significantly sanitize the codebase. This upgrade actually improves performance by about 7% across multiple dimensions.

No breaking changes, no new features, just better, simpler, easier to read code, better performance, and the foundation for more cleanups.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.65%. Comparing base (cdbc6a9) to head (e743716).
⚠️ Report is 12 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #554      +/-   ##
==========================================
+ Coverage   99.61%   99.65%   +0.03%     
==========================================
  Files         236      253      +17     
  Lines       28037    28299     +262     
==========================================
+ Hits        27930    28201     +271     
+ Misses         65       60       -5     
+ Partials       42       38       -4     

Flag	Coverage Δ
unittests	99.65% <100.00%> (+0.03%)	⬆️

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