Cut 19.4.0 and bump the package version in pyproject.toml,
CITATION.cff, and the uv.lock self-entry.

Minor: one new capability plus four help/UX fixes since 19.3.0.
Enum-ranged qualifier columns are now audited against their closed
Biolink vocabulary at build time, failing with
`qualifier-vocabulary-violation` (#198). The CLI help surface got a
sweep: `distill-weigh`'s `--policy` becomes a `Literal` with a full
docstring Args section (#199), `build-kg`/`validate`/`validate-kgx`
gain per-flag help matching docs/cli.md (#200), `build-fullmap`'s
shadowed `--version` flag is renamed to `--babel-version` (#201), and
phantom `--empty-*` list-reset flags no longer render in any help
(#202).

Changelog:
- Versioned the Unreleased section as 19.4.0, added the missing
  Fixed entries for #199, #200, and #202 with PR links.

Docs: none needed here; docs/cli.md and the flag docs shipped with
the feature PRs, and mkdocs builds strict.

Testing:
- make check -> All checks passed! (1663 passed, 52 skipped;
  rust: 127/11/17/1 ok)
- uv run --no-sync mkdocs build --strict -> pass