ci: test ubuntu-latest-4-cores runner with timing instrumentation

Probes whether larger runners are available to the org. If the job
queues and runs, mise parallelism should actually benefit from 4
real cores vs 2. If it fails, we know larger runners aren't enabled.

Adds nproc/RAM logging and SECONDS timing to measure the difference.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: scottschreckengaust