Remove the sign conversions and any differentiated handling

[mborland]

Closes: #388
Closes: #389

[cppalliance-bot]

An automated preview of the documentation is available at https://390.int128.prtest3.cppalliance.org/libs/int128/doc/html/index.html

If more commits are pushed to the pull request, the docs will rebuild at the same URL.

2026-04-30 15:29:05 UTC

[codecov]

Codecov Report

❌ Patch coverage is 60.09501% with 168 lines in your changes missing coverage. Please review.
✅ Project coverage is 95.91%. Comparing base (f5d7ca1) to head (a6e44c6).
⚠️ Report is 20 commits behind head on develop.

Files with missing lines	Patch %	Lines
test/test_builtin_parity.cpp	29.72%	0 Missing and 130 partials ⚠️
test/test_mixed_type_sign_compare.cpp	25.00%	0 Missing and 24 partials ⚠️
test/test_mixed_type_sign_conversion.cpp	22.22%	2 Missing and 12 partials ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #390      +/-   ##
===========================================
- Coverage    98.89%   95.91%   -2.99%     
===========================================
  Files           67       68       +1     
  Lines         5636     5827     +191     
  Branches      1788     1905     +117     
===========================================
+ Hits          5574     5589      +15     
- Misses          31       34       +3     
- Partials        31      204     +173     

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