Add formatting, docs-build and wheel-install checks to CI

[apdavison]

CI currently runs flake8 (syntax errors and undefined names only) and pytest. Three cheap additions:

• black --check . (the tree is already formatted, so this only prevents drift)
• a Sphinx build with -W, so a broken reference or malformed markup fails the pull request rather than surfacing later on the ReadTheDocs build
• building the wheel and importing it in a clean environment, so a packaging error (e.g., a missing subpackage in [tool.setuptools.packages.find]) cannot reach PyPI. CI only ever installs from the working tree with pip install -e .