[2/4] Split pull request CI by responsibility

[jlnav]

• Replaces the Python/OS/transport Cartesian matrix with independent static, unit, core, TCP, macOS, and coverage jobs.
• Uses Python 3.12 for canonical integration testing while retaining unit coverage on Python 3.12-3.14.
• Enables Pixi caching, fail-fast integration runs, and timing artifact uploads.
• Stack: parent test-runner/controls-and-timings; child [3/4] Tier and shard complete integration testing #1782.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.31%. Comparing base (0b9b296) to head (89611e8).

Additional details and impacted files

@@                         Coverage Diff                          @@
##           test-runner/controls-and-timings    #1783      +/-   ##
====================================================================
+ Coverage                             87.64%   89.31%   +1.66%     
====================================================================
  Files                                    83       83              
  Lines                                  8927     8927              
  Branches                               1384     1384              
====================================================================
+ Hits                                   7824     7973     +149     
+ Misses                                  866      718     -148     
+ Partials                                237      236       -1     

☔ 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.