## What this changes
## Behaviour change?
## How I tested it
## Anything reviewers should look at first?
## Checklist
- [ ] `cargo fmt --all` is clean (pre-commit hook enforces this).
- [ ] `cargo clippy -- -D warnings` is clean.
- [ ] `cargo test --bin hvac` passes locally.
- [ ] New behaviour has tests; bug fixes have regression tests.
- [ ] `CHANGELOG.md` updated if user-visible.
- [ ] Public items have `///` doc comments.
- [ ] No new dependencies, or the PR body says why one was added.