Cut 16.3.0 and bump the package version in pyproject.toml, uv.lock, and
CITATION.cff.

Minor: make loguru optional through the new [log] extra while retaining a
stdlib-backed logging fallback, remove redundant PMC .txt and .pdf
downloads, and drop the dead pdfminer.six agent dependency (#127, #128,
#129).

Changelog:
- Versioned Unreleased as 16.3.0 dated 2026-09-03.
- Added linked entries for #127 and #129.
- Linked the existing #128 entry and placed it under Changed.

Docs: none needed here; the merged changes already update the relevant
README, installation, development, contributor, Makefile, and CI docs.

Testing:
- uv run pytest -q -> 1122 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