Add new cohort_preserving_fixed charconv mode and fix non-rejection in cohort_preserving_scientific

[mborland]

Closes: #1410

[cppalliance-bot]

An automated preview of the documentation is available at https://1412.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.

2026-07-17 15:06:12 UTC

[codecov]

Codecov Report

❌ Patch coverage is 97.35099% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 98.3%. Comparing base (106ac50) to head (70e001b).

Files with missing lines	Patch %	Lines
include/boost/decimal/charconv.hpp	89.8%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #1412     +/-   ##
=========================================
- Coverage     98.3%   98.3%   -0.0%     
=========================================
  Files          291     291             
  Lines        23674   23822    +148     
  Branches      2070    2086     +16     
=========================================
+ Hits         23250   23395    +145     
- Misses         424     427      +3     

Files with missing lines	Coverage Δ
include/boost/decimal/decimal128_t.hpp	95.7% <ø> (ø)
include/boost/decimal/decimal32_t.hpp	95.8% <ø> (ø)
include/boost/decimal/decimal64_t.hpp	95.0% <ø> (ø)
include/boost/decimal/decimal_fast128_t.hpp	95.7% <ø> (ø)
include/boost/decimal/decimal_fast32_t.hpp	97.5% <ø> (ø)
include/boost/decimal/decimal_fast64_t.hpp	97.6% <ø> (ø)
include/boost/decimal/detail/from_chars_impl.hpp	100.0% <100.0%> (+3.2%)	⬆️
include/boost/decimal/detail/parser.hpp	92.2% <100.0%> (ø)
test/test_charconv_preservation.cpp	100.0% <100.0%> (ø)
include/boost/decimal/charconv.hpp	95.5% <89.8%> (-0.4%)	⬇️

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