Which of the run's two streams the default rendering goes to; output by default.

Separate from level because the two answer different questions: level is what kind of thing this is, which is what a consumer filters on, while this is where the CLI has always put it. A raw stack belongs on error alone — un-prefixed multi-line text on stdout would corrupt the TAP document — and a few diagnostics go to both deliberately, so a reader watching only one stream still sees them.