Fix nan handling and add non-standard payload function for reading

[mborland]

Closes: #1178

[cppalliance-bot]

An automated preview of the documentation is available at https://1180.decimal.prtest3.cppalliance.org/libs/decimal/doc/html/index.html

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

2025-11-03 16:22:52 UTC

[codecov]

Codecov Report

❌ Patch coverage is 93.75000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 99.3%. Comparing base (6a5da01) to head (fb51370).
⚠️ Report is 16 commits behind head on develop.

Files with missing lines	Patch %	Lines
include/boost/decimal/detail/cmath/nan.hpp	84.7%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #1180     +/-   ##
=========================================
- Coverage     99.3%   99.3%   -0.0%     
=========================================
  Files          255     255             
  Lines        16989   17014     +25     
  Branches      1907    1911      +4     
=========================================
+ Hits         16860   16883     +23     
- Misses         129     131      +2     

Files with missing lines	Coverage Δ
include/boost/decimal/decimal128_t.hpp	98.2% <ø> (ø)
include/boost/decimal/decimal32_t.hpp	96.6% <ø> (ø)
include/boost/decimal/decimal64_t.hpp	95.2% <ø> (ø)
test/test_cmath.cpp	100.0% <100.0%> (ø)
include/boost/decimal/detail/cmath/nan.hpp	85.8% <84.7%> (-14.2%)	⬇️

... and 1 file with indirect coverage changes

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