Cut 16.1.0 and bump the package version in pyproject.toml, uv.lock, and
CITATION.cff.

Minor: two new capabilities ship since 16.0.0. Release mode now drops
applied_to_treat edges whose case_count is below 25 (drop_low_case_count,
gated on --release and the applied_to_treat predicate, null counts kept,
runs in the significance phase before entity resolution) (#123). build-kg
gains --no-original/-no, which omits original_subject, original_object,
and any other original_* fields from the final edge NDJSON while keeping
them mid-pipeline for resolution and QC audits; default output is
unchanged (#124).

Changelog:
- Created the 16.1.0 - 2026-09-03 section from #123 and #124 (no
  Unreleased section existed), wrote both entries in the existing style
  with their PR links.

Docs: none needed here. #124 documented --no-original in docs/cli.md when
it merged, and this release ships no behavior beyond those two PRs.

Testing:
- uv run pytest -q -> 1112 passed, 15 skipped (94% coverage)
- uv run ruff check . && uv run ruff format --check . && uv run pyright -> clean
- uv lock --check -> up to date
- uv run mkdocs build --strict -> clean