Cut 16.4.0 and bump the package version in pyproject.toml, uv.lock, and
CITATION.cff.

Minor: add the autonomous agent's default 50-row table and worksheet guard
with the configurable --min-rows/-mr threshold and fail-open handling
(#131); keep pow and copysign math transformations lazy with native Polars
expressions (#130); and add a PyPI monthly-download badge to the README
(#133).

Changelog:
- Added the 16.4.0 section with linked entries for the agent small-table
  guard, native Polars math transformations, and PyPI downloads badge.

Docs: docs/agent.md, docs/cli.md, and README.md were updated by the shipped
changes; no additional release documentation was needed here.

Testing:
- uv run pytest -q -> 1139 passed, 15 skipped (94% coverage)
- uv run ruff check . && uv run ruff format --check . && uv run pyright
  -> clean / 0 errors
- uv lock --check -> up to date
- uv run mkdocs build --strict -> clean