Add a CI workflow that runs the test suite on every PR #9

The repo has no .github/workflows/, so nothing runs the suite on a PR. #7 merged on a local run only (uv run pytest -q, 262 passed), and #3's body says the same ("No CI in this repo; the gate was uv run pytest -q locally").

There is no Makefile, so the minimal workflow is uv sync --locked + uv run pytest -q on ubuntu-latest (public repo), triggered on push to main and on every pull_request with no branches: filter.

The tests shell out to poppler (pdftotext, pdfinfo, pdffonts in test_citations.py, test_book.py, test_furniture.py, …), so the job also needs poppler-utils. WeasyPrint needs Pango, which ubuntu-latest ships, but that is worth confirming on the first run rather than assuming.