chore: prepare 4.0.1 release (#440)

## Summary

Bumps `zfs-replicate` to 4.0.1 and promotes the `unreleased` CHANGELOG
section. The only user-visible change since 4.0.0 is the
snapshot-equality fix from #390 (landed via #439).

## Gotchas

- After merge, the release still has to be cut by hand (tag + GitHub
Release) since release-please isn't wired up yet (#415). Publishing then
runs automatically via `.github/workflows/publish.yml` on the `release:
published` event.
- `PYPI_API_TOKEN` is from 2022-12-11; worth a sanity check before
tagging. Stale `PYPI_USERNAME` / `PYPI_PASSWORD` secrets from 2021 are
unused by the workflow and can be deleted post-release.

Closes #390 milestone work (issue itself was closed by #439).

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Merge pull request #352 from alunduil/release

Bump version to 4.0.0

Merge pull request #300 from alunduil/release

Bump version from 3.2.12 to 3.2.13.

Merge pull request #296 from alunduil/release

Bump version from 3.2.11 to 3.2.12.

Merge pull request #291 from alunduil/release

Bump version from 3.2.10 to 3.2.11.

Merge pull request #285 from alunduil/release

Bump version from 3.2.9 to 3.2.10.

Merge pull request #280 from alunduil/release

Bump version from 3.2.8 to 3.2.9.

Merge pull request #276 from alunduil/release

Bump version from 3.2.7 to 3.2.8.

Merge pull request #273 from alunduil/release

Bump version from 3.2.6 to 3.2.7.