Cut 19.3.0 and bump the package version in pyproject.toml,
CITATION.cff, and uv.lock.

Minor: the engine migrates to polars 2.0 with full output parity (#197):
all lazy queries run on the 2.0 streaming engine by default, `pyarrow`
joins the core dependencies, and `fastexcel` is bumped to >=0.21.0,
while headerless-source column naming, NLP tier selection, and KGX
NDJSON emission order stay byte-for-byte stable. The README, docs
landing page, and llms.txt front door is corrected and restructured to
match the shipped CLI and site nav (#196).

Changelog:
- Versioned the Unreleased section as 19.3.0, added the missing #196
  front-door entry, and dated the release.

Docs: none needed here (the shipped docs surface is itself what #196
fixed, and the changelog entry documents it).

Testing:
- make lint -> All checks passed
- make fmt-check -> 100 files already formatted; cargo fmt clean
- make typecheck -> 0 errors, 0 warnings (after syncing local venv to
  CI's env: --group dev --extra cli --extra qc --extra log)
- uv run pytest -q -> 1651 passed, 52 skipped in 29.20s
- cargo test (rust/) -> all suites ok, 0 failed
- cargo clippy --all-targets -- -D warnings -> clean
- uv lock --check -> ok
- uv run mkdocs build --strict -> built in 0.46s