[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 (8698541) to head (1decffa).

Additional details and impacted files

@@                         Coverage Diff                          @@
##           test-runner/controls-and-timings    #1783      +/-   ##
====================================================================
- Coverage                             89.33%   89.31%   -0.03%     
====================================================================
  Files                                    83       83              
  Lines                                  8927     8927              
  Branches                               1384     1384              
====================================================================
- Hits                                   7975     7973       -2     
- Misses                                  717      718       +1     
- Partials                                235      236       +1     

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