One concurrent group of a run: the files bundled into a single page, and where that page's artifacts were written.
Which files land together is decided per run from recorded timings and the core count, so it is neither stable across runs nor derivable by the caller — and it is exactly what you need to reproduce a failure that only happens when two files share a bundle.
const group: RunGroup = { index: 1, files: ['/proj/test/cart.ts'], output: '/proj/tmp/group-1' }; group.output; // '/proj/tmp/group-1' — still on disk after the run, unlike the group's URL