Add example of rollover behavior

[mborland]

Closes: #313

[cppalliance-bot]

An automated preview of the documentation is available at https://314.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.

2026-02-04 19:29:00 UTC

[codecov]

Codecov Report

❌ Patch coverage is 73.33333% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 99.02%. Comparing base (eadfd90) to head (77a5ac9).
⚠️ Report is 4 commits behind head on develop.

Files with missing lines	Patch %	Lines
examples/rollover.cpp	73.33%	0 Missing and 4 partials ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #314      +/-   ##
===========================================
- Coverage    99.09%   99.02%   -0.08%     
===========================================
  Files           60       61       +1     
  Lines         5303     5318      +15     
  Branches      1633     1643      +10     
===========================================
+ Hits          5255     5266      +11     
  Misses          21       21              
- Partials        27       31       +4     

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