why: Enable parallel pytest runs via -n auto, cutting local test
turnaround. Adds the dependency only — opt-in via pytest CLI flags;
no addopts changes.
what:
- Add pytest-xdist to dev and testing dependency groups
- Lock with uv lock (transitively adds execnet)

Re-ports PR #930 against the current uv-based dependency layout
(original PR targeted poetry.lock, which has since been migrated
to uv.lock).

See also:
- https://pypi.org/project/pytest-xdist/
- https://github.com/pytest-dev/pytest-xdist