Fix fmod post-condition check for negative remainders

[0x74h51N]

Fixes #764

The post-condition added in #605 tests |r| >= |b|, but does it with a signed compare against an operand matched to the remainder's sign. That equivalence only holds while both values are positive; once the remainder is negative the ordering reverses and every valid remainder satisfies it, so the correction fires on every call and adds one |b|. What comes back is the Euclidean remainder instead of the truncated one.

The comparison is now read by magnitude. The branch structure and the subtraction directions are unchanged.

git_issue_604.cpp covered all four sign combinations, but only with a zero remainder, where both conventions agree, so it could not separate them. It now also covers non-zero remainders and |a| < |b|.

This keeps fmod on std::fmod semantics, which is what float128 and mpfr_float still return.

[ckormanyos]

Hi @0x74h51N I just approved your workflow run. In this repo first-time contributors need workflow approval run. I see you also added relevant tests to this issue resolution. Thank you for those as well.

Let's see how CI runs.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.2%. Comparing base (14ba272) to head (16b57d0).

Additional details and impacted files

@@            Coverage Diff            @@
##           develop    #771     +/-   ##
=========================================
+ Coverage     96.2%   96.2%   +0.1%     
=========================================
  Files          302     302             
  Lines        29478   29512     +34     
=========================================
+ Hits         28344   28378     +34     
  Misses        1134    1134             

Files with missing lines	Coverage Δ
...nclude/boost/multiprecision/detail/default_ops.hpp	93.6% <100.0%> (+0.1%)	⬆️
test/git_issue_604.cpp	100.0% <100.0%> (ø)

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[ckormanyos]

Hi @0x74h51N the CI run went green. There are two missed coverage lines. But we can fill in more tests later. This is better than the previous state. So I am going to merge to develop now.

Thank you very much for looking into this and improving the logic.

Cc: @jzmaddock and @mborland and @cosurgi