Change encoding of fast non-finite values to allow for payloads

[mborland]

Closes: #1181

[cppalliance-bot]

An automated preview of the documentation is available at https://1182.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.

2025-11-04 10:38:02 UTC

[codecov]

Codecov Report

❌ Patch coverage is 97.50000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 99.3%. Comparing base (e3a86a8) to head (6094c8d).
⚠️ Report is 31 commits behind head on develop.

Files with missing lines	Patch %	Lines
include/boost/decimal/detail/cmath/nan.hpp	93.4%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #1182     +/-   ##
=========================================
- Coverage     99.3%   99.3%   -0.0%     
=========================================
  Files          255     255             
  Lines        17014   17049     +35     
  Branches      1911    1915      +4     
=========================================
+ Hits         16884   16918     +34     
- Misses         130     131      +1     

Files with missing lines	Coverage Δ
include/boost/decimal/decimal_fast128_t.hpp	96.0% <100.0%> (ø)
include/boost/decimal/decimal_fast32_t.hpp	100.0% <100.0%> (ø)
include/boost/decimal/decimal_fast64_t.hpp	100.0% <100.0%> (ø)
test/test_cmath.cpp	100.0% <100.0%> (ø)
include/boost/decimal/detail/cmath/nan.hpp	92.9% <93.4%> (+7.2%)	⬆️

... and 1 file with indirect coverage changes

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