Drives `borg benchmark cpu --chunking --json` once per scan kernel
combination and prints one throughput table over all chunkers and all
kernels this build and CPU can run, which is how the per-platform kernel
defaults were and are decided (#10160).
`borg benchmark cpu` reports a chunker it cannot construct - e.g. because
an unusable kernel was requested - as an {"error": ...} row and carries on
with exit code 0, so the script treats such a row as the run's failure;
that is also how it discovers which kernels exist (an invalid name makes
borg list the valid ones).
Co-Authored-By: Claude Fable 5.1 <[email protected]>
