Cut 18.0.0 and bump the package version in pyproject.toml,
CITATION.cff, and uv.lock.

Major: remove the deprecated --no-original build-kg option and API
parameters; original_* edge values are now always retained and
deterministically collision-merged. Add canonical regulatory approvals
overrides, content-aware section-store invalidation, and the associated
documentation and regression coverage.

Changelog:
- Versioned the Unreleased section as 18.0.0 - 2026-09-10.
- The release section retains the merged feature and documentation
  entries already landed on main.

Docs: none needed here; the user-facing documentation changes are
already included in the released Unreleased section.

Testing:
- make check -> passed
- uv run ruff check . -> All checks passed
- uv run ruff format --check . -> 88 files already formatted
- uv run pytest -> 1324 passed, 43 skipped
- cargo test --manifest-path rust/Cargo.toml -> 136 passed, 1 ignored
- cargo clippy --manifest-path rust/Cargo.toml --all-targets -- -D warnings
  -> passed
- uv run pyright -> passed