Introduce pyproject.toml with uv workspace

Introduce pyproject.toml with uv workspace

Add a root pyproject.toml that defines a uv workspace over the existing
packaging/ directory and declares PEP 735 dependency groups for development
tools (test, lint, types, docs, serve).
These groups are ignored by pip and only activated when using uv, so the
existing virtualenv + pip workflow is unaffected. The 'dev' group includes
all sub-groups by default, so 'uv sync' installs everything needed for
development in one command.