Test and fix support for iostream manipulators

[mborland]

Closes: #146
Closes: #147
Closes: #148
Closes: #149
Closes: #150
Closes: #151

[cppalliance-bot]

An automated preview of the documentation is available at https://152.safe-numbers.prtest3.cppalliance.org/libs/safe_numbers/doc/html/index.html

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

2026-03-24 18:41:03 UTC

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.6%. Comparing base (df39abb) to head (2b06082).
⚠️ Report is 5 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop    #152     +/-   ##
=========================================
+ Coverage     97.6%   97.6%   +0.1%     
=========================================
  Files          112     112             
  Lines         7942    8169    +227     
  Branches      2976    3048     +72     
=========================================
+ Hits          7745    7972    +227     
  Misses         131     131             
  Partials        66      66             

Files with missing lines	Coverage Δ
test/test_unsigned_streaming.cpp	100.0% <100.0%> (ø)

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