Notebook test

[therooler]

Closes #14 by converting notebooks to scripts and running them with pytest.

Proposed changes

Adds automated end-to-end execution tests for the example notebooks (docs/examples/*.ipynb) using nbconvert, so an API change that breaks a documented example is caught in CI rather than by users. The notebook tests are heavy, so they're kept out of the standard test suite and run in their own dedicated workflow — triggered on notebook changes and reused by the release pipeline to gate publishing.

Details

• tests/test_notebooks.py auto-discovers every docs/examples/*.ipynb and re-executes each from a clean state via nbconvert's ExecutePreprocessor (900s timeout, run with the notebook's own dir as cwd). A test fails if any cell raises, surfacing the offending cell's traceback. Test IDs are the notebook filenames, so new examples are picked up automatically.

• The module is marked pytest.mark.notebooks; pyproject.toml registers the notebooks marker and sets addopts = "-m 'not notebooks'", so a bare pytest (and the existing tests.yml job) deselects them.
  Added notebook-execution deps (nbconvert, nbformat, ipykernel, matplotlib) to the [dev] extra.
  Dedicated reusable workflow

• .github/workflows/notebooks.yml runs pytest -m notebooks -v. Triggers: pull_request/push to main filtered to paths that affect notebooks (docs/examples/**.ipynb, tests/test_notebooks.py, pyproject.toml, the workflow itself).

• updated .github/workflows/release.yml to call the reusable workflow via a test-notebooks job, and publish-testpypi now needs: [test-install, test-notebooks] so a broken notebook blocks the release.

• pytest and tests.yml are unchanged in behavior (notebooks deselected).

• pytest -m notebooks selects and runs both example notebooks; both pass locally (pulse_smoothing.ipynb ~2.5 min after the fix).