Add spaceship operator support for both int128_t and uint128_t

[mborland]

Closes: #308

[cppalliance-bot]

An automated preview of the documentation is available at https://309.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.

2026-02-02 18:28:53 UTC

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.90%. Comparing base (c7f1de1) to head (d3c5ac7).
⚠️ Report is 8 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #309   +/-   ##
========================================
  Coverage    98.89%   98.90%           
========================================
  Files           59       60    +1     
  Lines         5262     5290   +28     
  Branches      1614     1626   +12     
========================================
+ Hits          5204     5232   +28     
  Misses          31       31           
  Partials        27       27           

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