Fix CI coverage for the src layout

[ryanduguid]

Summary

The coverage job still targets ./pytest_order/, which does not exist with the src layout. Select the installed pytest_order package and map Linux and Windows Tox paths back to src/pytest_order so coverage XML refers to tracked files.

Evidence

• Before: all six Python 3.13 Tox environments pass 142 tests on Linux, but collect no coverage data and produce no XML.
• After: the same six environments pass 142 tests each on Linux and Windows. Every XML report includes all five tracked package files, with 668/671 lines and 277/286 branches covered.
• Installed package contents match the checked source. Applicable pre-commit hooks pass; Python hooks skip these configuration files. git diff --check passes.

Merge danger

Door: two-way. Blast radius: CI.

The change is limited to coverage selection and path mapping. Security scans retain two existing workflow warnings: checkout credential persistence and the unpinned Codecov action.

Unverified

Hosted GitHub Actions and Codecov processing, other Python versions, PyPy and macOS. Full-repository pre-commit, documentation builds and standalone distribution checks were not run.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.21%. Comparing base (7d0355a) to head (fad8f89).

Additional details and impacted files

@@            Coverage Diff            @@
##           main     #266       +/-   ##
=========================================
+ Coverage      0   98.21%   +98.21%     
=========================================
  Files         0        5        +5     
  Lines         0      671      +671     
  Branches      0      143      +143     
=========================================
+ Hits          0      659      +659     
- Misses        0        3        +3     
- Partials      0        9        +9     

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[mrbean-bremen]

Thanks!