Fix std::showbase vs std::noshowbase

[mborland]

Closes: #370

[cppalliance-bot]

An automated preview of the documentation is available at https://371.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.

2026-03-24 18:30:02 UTC

[codecov]

Codecov Report

❌ Patch coverage is 95.23810% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 98.90%. Comparing base (6b4f7a5) to head (5517b6b).
⚠️ Report is 4 commits behind head on develop.

Files with missing lines	Patch %	Lines
include/boost/int128/iostream.hpp	83.33%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #371      +/-   ##
===========================================
- Coverage    98.91%   98.90%   -0.02%     
===========================================
  Files           66       66              
  Lines         5538     5551      +13     
  Branches      1757     1767      +10     
===========================================
+ Hits          5478     5490      +12     
- Misses          29       30       +1     
  Partials        31       31              

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