Optimize further mul and div algorithms

[mborland]

Closes: #410

[cppalliance-bot]

An automated preview of the documentation is available at https://411.int128.prtest3.cppalliance.org/libs/int128/doc/html/index.html

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

2026-06-03 15:00:51 UTC

[codecov]

Codecov Report

❌ Patch coverage is 93.23529% with 23 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.19%. Comparing base (051606b) to head (bf76e1d).
⚠️ Report is 24 commits behind head on develop.

Files with missing lines	Patch %	Lines
test/test_div_primitives.cpp	86.08%	2 Missing and 14 partials ⚠️
include/boost/int128/detail/common_div.hpp	95.45%	3 Missing and 2 partials ⚠️
test/test_x64_msvc_div.cpp	95.34%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #411      +/-   ##
===========================================
- Coverage    96.28%   96.19%   -0.10%     
===========================================
  Files           78       79       +1     
  Lines         6999     7068      +69     
  Branches      2397     2450      +53     
===========================================
+ Hits          6739     6799      +60     
+ Misses          41       34       -7     
- Partials       219      235      +16     

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