feat: graceful guest drain on systemctl stop / host shutdown (#412)

* fix: drop removed predastore s3.Config.Compaction from admin_test

The predastore submodule bump removed the Compaction field from s3.Config.
Parse the rendered [compaction] block into a local struct so the template
round-trip test no longer references the deleted field.

* feat: graceful guest drain on systemctl stop / host shutdown

systemctl stop spinifex.target tore down nbdkit/viperblock while guest QEMU
was still writing, corrupting guest filesystems. The ordered drain existed
only over NATS via cluster shutdown, never on the systemd/host path.

Add spx admin node drain --local (GATE+DRAIN against the local node, leaving
STORAGE/PERSIST/INFRA to systemd), wired as the ExecStop of a new
spinifex-shutdown.service oneshot ordered After= all services so it stops
first and drains guests while every service is still up. DDIL preserved: the
oneshot fires only on target/host shutdown, not on a daemon restart.

Harden the viperblockd SIGTERM handler to flush the WAL but skip reaping an
nbdkit a guest is still attached to. Add a units ordering invariant.

* fix: ExecStop drain timeout needs duration unit (120 -> 120s)

The spinifex-shutdown.service ExecStop passed --timeout=120, but spx admin
node drain's --timeout is a cobra Duration flag and rejects a bare integer
(time: missing unit in duration "120"). The drain exited 1 without running,
so a systemctl stop tore nbdkit out from under the running guest and corrupted
its ext4 (the exact failure this feature prevents). Pass 120s.

* fix: nbdkitInUse read ss state from correct column + cover it

ss -H rows are '<netid> <state> ...', so the state is field[1], not field[0]
(field[0] is 'u_str'). The old index never matched ESTAB, so an in-use nbdkit
read as idle and the SIGTERM path would reap it under a live guest. Add tests
exercising a real connected unix socket, an absent socket, and TCP transport.

docs/COMMANDS.md

@@ -66,6 +66,7 @@ Operational commands for inspecting cluster state. These fan out NATS requests t
 |---------|-------|-------------|
 | `spx admin cluster shutdown` | `--force` (shutdown even if nodes don't respond), `--timeout` (max wait per phase, default 120s), `--dry-run` (print phase plan without executing) | Performs coordinated, phased shutdown of entire cluster. Phases execute in order: GATE (stop API/UI) → DRAIN (stop VMs) → STORAGE (stop viperblock) → PERSIST (stop predastore) → INFRA (stop NATS/daemon). Each phase waits for all nodes to ACK before proceeding. Uses JetStream state tracking. |
 | `spx admin cluster drain-dhcp` | `--timeout` (reply-collection window, default 30s) | Asks each vpcd to DHCPRELEASE every external-pool DHCP lease it currently holds, returning them to the upstream DHCP server. Run on teardown before stopping services — an env reset otherwise strands held leases upstream until their TTL expires, eventually exhausting the upstream scope. Best-effort: warns and exits 0 if the cluster is already down. |
+| `spx admin node drain --local` | `--local` (drain the local node only, required), `--timeout` (max wait per phase, default 120s) | Runs the GATE and DRAIN phases against the local node only: powers down its guests via QMP and unmounts their volumes (flushing the viperblock WAL) while every service is still up. STORAGE/PERSIST/INFRA are left to systemd's ordered teardown. Wired as the `ExecStop` of `spinifex-shutdown.service`, so `systemctl stop spinifex.target` and host reboot/poweroff drain guests before any storage service stops. |
 
 ### Certificate Management