Allow explicit operations only between int128_t and uint128_t

[mborland]

Closes: #337
Closes: #338

[cppalliance-bot]

An automated preview of the documentation is available at https://340.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.

2026-03-02 23:47:10 UTC

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.05%. Comparing base (f7dc6f2) to head (dd01ce0).
⚠️ Report is 16 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #340      +/-   ##
===========================================
+ Coverage    99.03%   99.05%   +0.02%     
===========================================
  Files           64       66       +2     
  Lines         5402     5526     +124     
  Branches      1688     1742      +54     
===========================================
+ Hits          5350     5474     +124     
  Misses          21       21              
  Partials        31       31              

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