Restructure CI coverage uploads and add codecov.yml

[blink1073]

References

Description

Limits Codecov uploads to two canonical jobs (Linux and Windows on Python 3.10) and adds a codecov.yml to configure reporting behavior.

Changes

• Added a dedicated cover job (matrix: ubuntu-latest, windows-latest, Python 3.10) that runs just cover and uploads to Codecov
• Excluded ubuntu-latest/3.10 and windows-latest/3.10 from the main test matrix; those jobs now run just test only
• Added python-version: ${{ matrix.python-version }} to the setup-uv step in the test job
• Added codecov.yml with after_n_builds: 2, project/patch coverage targets (90%/95%), and PR comment layout

Backwards-incompatible changes

None

Testing

CI will validate on push.

AI usage

• Some or all of the content of this PR was generated by AI.
• The human author has carefully reviewed this PR and run this code.
• AI tools and models used: Claude Sonnet 4.6 (Claude Code)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.85%. Comparing base (426bb2d) to head (a35a4ce).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #381   +/-   ##
=======================================
  Coverage   90.85%   90.85%           
=======================================
  Files          51       51           
  Lines        2952     2952           
  Branches      413      413           
=======================================
  Hits         2682     2682           
  Misses        190      190           
  Partials       80       80           

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