Add Python 3.14 to CI matrix

[mgoldsborough]

Summary

The CI currently only tests against Python 3.13. Add Python 3.14 to the test matrix so we catch compatibility issues early.

What to do

1. Edit .github/workflows/ci.yml
2. Change the lib job to use a strategy matrix:

   strategy:
     matrix:
       python-version: ["3.13", "3.14"]

3. Update actions/setup-python to use ${{ matrix.python-version }}
4. Verify CI passes on both versions

Acceptance criteria

• CI runs tests on both Python 3.13 and 3.14
• Both versions pass

Estimated time: ~30 minutes