Replace int128_t high word with uint64_t

[mborland]

This allows proper vectorization but requires massive refactoring. Also includes auto-generated benchmarks so that we can keep things current more easily.

Closes: #465

[cppalliance-bot]

An automated preview of the documentation is available at https://469.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.

2026-07-28 15:45:09 UTC

[codecov]

Codecov Report

❌ Patch coverage is 87.50000% with 30 lines in your changes missing coverage. Please review.
✅ Project coverage is 95.81%. Comparing base (0a7509b) to head (d2a9813).
⚠️ Report is 19 commits behind head on develop.

Files with missing lines	Patch %	Lines
test/test_builtin_parity.cpp	56.92%	0 Missing and 28 partials ⚠️
test/github_issue_432.cpp	60.00%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #469      +/-   ##
===========================================
- Coverage    96.12%   95.81%   -0.31%     
===========================================
  Files           87       88       +1     
  Lines         7566     7675     +109     
  Branches      2739     2792      +53     
===========================================
+ Hits          7273     7354      +81     
  Misses          44       44              
- Partials       249      277      +28     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.