Fix shortest round-trip floating-point stringification

[sb123sb123]

Fixes #3197.

The default floating-point stringifier currently emits fixed notation with max_digits10 digits after the decimal point, which can make nearby double values indistinguishable in failure messages. For the default round-trip precision, this change uses defaultfloat and selects the first significant-digit precision that round-trips, preserving the sign of zero. Non-default precision values retain the existing fixed-format behavior, and infinities are handled separately.

Adds a regression test for the adjacent values 5.000000000000001e-05 and 5e-05, and updates the affected approval baselines.

Validation: MSVC 19.43 x64 Release build of SelfTest; CTest passed 82/82 tests, including approval tests. clang-format passed on the changed C++ hunks.

[codecov]

Codecov Report

❌ Patch coverage is 78.94737% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.11%. Comparing base (222e233) to head (e754f31).

Additional details and impacted files

@@            Coverage Diff             @@
##            devel    #3208      +/-   ##
==========================================
- Coverage   91.17%   91.11%   -0.06%     
==========================================
  Files         206      206              
  Lines        9080     9099      +19     
==========================================
+ Hits         8278     8290      +12     
- Misses        802      809       +7     

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[sb123sb123]

AppVeyor build 5688’s only failed job is the VS 2017 Release Coverage job. Its log shows all 81 CTest MemCheck tests passed, then the job exited 1 because codecov was not recognized; the other four AppVeyor jobs passed. This looks like a coverage-uploader/environment failure after the tests, not a test failure. Could this check be retried or the uploader step be handled for this PR? AppVeyor log