Add Python 3.15 (rc1) to the CI matrices

[woutervh]

Summary

Python 3.15rc1 is out. Adds "3.15" to both the testing.yaml and linting.yaml
matrices, ahead of the stable release.

Verified locally first (not just assumed):

• uv python install "3.15" resolves to 3.15.0rc1 - there's no stable 3.15 release yet,
  so uv treats the rc as the match for the bare "3.15" version string (same format
  already used for every other matrix entry).
• Ran the full test-suite against that interpreter in an isolated venv (not the project's
  own .venv, to avoid clobbering it): all 21 tests pass, unmodified, no compatibility
  code needed.

Per discussion, this is a normal matrix entry (no continue-on-error) - a regression on
the rc fails the workflow run just like any other Python version would.

Testing

• pytest: 21 passed, 100% coverage (>= 90% required), against both 3.14 (project venv)
  and 3.15.0rc1 (isolated venv)
• ruff check .: clean
• ty check: clean

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (264a839) to head (b562b52).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #127   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            4         4           
  Lines           69        69           
  Branches         3         3           
=========================================
  Hits            69        69           

Flag	Coverage Δ
unittests	100.00% <ø> (ø)

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