Support progress bars

- [ ] Render and update a progress bar for humans
- will probably need to return something non-`()` from `Output::print`
- [ ] Render other stuff above it while progress bar is visible
- [ ] Render usable stuff to JSON (e.g. a 'tick' with current state each _n_ seconds)

Start by reading the `Output::print` entry point mentioned in the issue and review the three unchecked requirements. Determine how human progress updates, output above a visible bar, and periodic JSON state should behave. Done means all listed progress-bar and JSON-output behaviors are implemented and verifiable.

Written by the indexing model from the issue text.