feat(suidhelper): run the firecracker jailer through the setuid helper#47
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The BEAM no longer invokes the jailer binary directly. It calls the helper with a new `jailer` subcommand passing only untrusted-origin values (--id, --uid, --gid, repeated --cgroup KEY=VALUE, --api-sock); the helper reads the firecracker/jailer binary paths, chroot base, parent cgroup and accepted uid/gid band from its trusted /etc/hyper/config.toml, validates the caller's args, re-acquires permanent root, and execve's the jailer in place (same pid, so MuonTrap.Daemon keeps supervising it). Config: the helper's tool paths move into a `[tools]` table (joining the existing dmsetup/losetup/blockdev with new firecracker/jailer keys) and jail placement into `[jails]` (cgroup, uid_gid_range) -- matching the namespaces the Elixir node already reads. firecracker/jailer have no default and surface BinError::Unconfigured when absent; a present uid_gid_range with min==0 or min>max is fatal at load. Adds setuid_privileged::become_root_permanently (seal real+effective+saved uid/gid to 0, reset supplementary groups) for the execve handoff -- no Drop guard because execve replaces the image and the jailer drops its own privs. jailer.rs validates every caller value with refusal-first proptests (vm_id, id range, cgroup KEY=VALUE, api-sock shape). Elixir Jailer.command/2 emits the trimmed argv against Cfg.Tools.suidhelper().
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