Add support for C++26 constexpr std::format

[mborland]

Closes: #428

[cppalliance-bot]

An automated preview of the documentation is available at https://462.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.

2026-07-24 18:55:13 UTC

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.89%. Comparing base (3382905) to head (53ad84c).
⚠️ Report is 25 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #462      +/-   ##
===========================================
- Coverage    95.99%   95.89%   -0.11%     
===========================================
  Files           83       85       +2     
  Lines         7323     7397      +74     
  Branches      2595     2625      +30     
===========================================
+ Hits          7030     7093      +63     
  Misses          54       54              
- Partials       239      250      +11     

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