nan-vmd: memory/pids cgroup for the e2fsprogs sandbox #146

Follow-up from the security review of helmcode/nan-vmd#72 (workspace backups, #130). The restore sandbox (internal/sandbox) is bounded only by rlimits (1024 processes x 8 GiB address space, plus memfd) and by the nan-vmd pod cgroup. A compromised e2fsprogs could exhaust the nan-vmd pod memory. Put each sandbox run in a child cgroup with memory.max and pids.max. Also: per-VM uid instead of the shared fallback JailUID for root-owned disks (RLIMIT_NPROC is per uid), drop the unused "tmp" from the probe allowlist, and bind /dev/{null,zero,urandom} read-only.

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