Add increment and decrement for signed integers

[mborland]

Closes: #130

[cppalliance-bot]

An automated preview of the documentation is available at https://165.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.

2026-04-07 16:10:12 UTC

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.9%. Comparing base (9b638ff) to head (d8d624e).
⚠️ Report is 5 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop    #165     +/-   ##
=========================================
+ Coverage     96.8%   96.9%   +0.1%     
=========================================
  Files          120     121      +1     
  Lines         9260    9356     +96     
  Branches      3507    3537     +30     
=========================================
+ Hits          8962    9058     +96     
  Misses         232     232             
  Partials        66      66             

Files with missing lines	Coverage Δ
...boost/safe_numbers/detail/signed_integer_basis.hpp	74.6% <100.0%> (+2.0%)	⬆️
test/test_signed_inc_dec.cpp	100.0% <100.0%> (ø)

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