feat(esh-pve-nas): cut PVE root over to ZFS on the mirrored NVMe

Root is now nvme/ROOT/pve-1. The USB DOM keeps the ESP and /boot but is
out of the runtime I/O path, so a bus reset can no longer drop root from
under a running hypervisor. All five guests healthy, three pools ONLINE,
system running, ext4 pve-root intact and unmounted as the rollback with
its own kernel and initrd. zfs-import-cache is now the active import
path -- the all-three-pools cachefile fix doing its job.

The window cost an unplanned outage, and the cause was this repo's own
tooling rather than the migration.

The staging chroot ran `mount --rbind /dev` and /sys with no
--make-rslave. On systemd `/` has shared propagation, so the cutover's
`umount -R` propagated back into the live host and removed the real
/sys/fs/cgroup, /dev/pts and /dev/shm. With cgroup2 gone systemd-logind
could not create a session: ping fine, TCP fine, SSH authentication
succeeded, resident daemons kept serving -- and every new exec hung,
including /sbin/reboot, so the reboot never ran at all.

It impersonates failing root-disk I/O almost perfectly, and I called it
as the DOM dying. That was wrong. dmesg had the answer throughout: the
DOM attached cleanly with no errors, and the last log timestamp was
12114881s -- 140 days -- meaning this was still the original boot. A
down-detector had also never reported the host down, which I read as a
fast reboot rather than as no reboot.

Fixes and guards:

- --make-rslave after every rbind, plus a guard that refuses to proceed
  while any chroot bind still reports shared propagation.
- Confirm a reboot by observing the host DOWN, not by watching for it to
  come back. Those two states are indistinguishable otherwise.
- Blast radius now measured from the server: `ss` inside CT 103 found
  five NFS clients, not the two documented. The new one that mattered is
  esh-vm-db, hard-mounted and unreachable by ssh. Left mounted on
  purpose and it came through read-write.
- grub-reboot's one-shot does NOT work here: grubenv sits on an LVM LV
  which GRUB reads but cannot write, so next_entry survived the boot
  that consumed it. Steady state is saved_entry=pve-zfs-root with no
  next_entry. There is no auto-fallback on this host and no IPMI.

Recovery needed no console: an idempotent cgroup2/devpts/shm remount
landed in the brief windows where exec succeeded. No data was lost, and
neither the DOM nor any pool was ever at risk.

playbooks/esh-cutover-1-quiesce-docker-vm.yaml

@@ -0,0 +1,55 @@
+# esh-pve-nas cutover, step 1 of 5 — quiesce esh-docker-vm's hard NFS mounts.
+#
+# Run:  scripts/elway infra-ops@10.0.50.45 --playbook playbooks/esh-cutover-1-quiesce-docker-vm.yaml
+#
+# Why this is first and why it is not optional: /mnt/books and /mnt/backup are
+# `hard` NFS from CT 103 on esh-pve-nas. A hard mount does not fail when the
+# server goes away — it blocks forever in D-state, and the only known remedy is
+# rebooting THIS host. /mnt/books was deliberately left hard because calibre's
+# SQLite risks corruption under `soft`, so the mount option is not the fix; the
+# quiesce is.
+#
+# Measured 2026-08-18: exactly one container binds these paths
+# (calibre-web-automated -> /mnt/books/calibre/{ingest,calibre_library}) and
+# /mnt/backup has no container consumers at all. The blast radius is one service,
+# not the seventeen containers on this host.
+#
+# Reversed by playbooks/esh-cutover-5-restore.yaml.
+
+steps:
+  # No --format here: elway substitutes {{ ... }}, so Go template braces in a
+  # shell command are a booby trap. --filter + -q avoids them entirely.
+  - name: Stop the only container holding the NFS mounts
+    shell: sudo -n docker stop calibre-web-automated
+    when: "test -n \"$(sudo -n docker ps -q --filter name=^calibre-web-automated$)\""
+
+  - name: Confirm nothing else has files open under the mounts
+    shell: |
+      busy=$(sudo -n lsof +D /mnt/books +D /mnt/backup 2>/dev/null | tail -n +2 | wc -l)
+      if [ "$busy" -ne 0 ]; then
+        echo "STILL BUSY — refusing to unmount:"
+        sudo -n lsof +D /mnt/books +D /mnt/backup 2>/dev/null | head -20
+        exit 1
+      fi
+      echo "no open files under either mount"
+    changed_when: "false"
+
+  - name: Unmount /mnt/books
+    shell: sudo -n umount /mnt/books
+    when: "mountpoint -q /mnt/books"
+
+  - name: Unmount /mnt/backup
+    shell: sudo -n umount /mnt/backup
+    when: "mountpoint -q /mnt/backup"
+
+verify:
+  - name: Neither NFS mount remains
+    shell: "! findmnt -t nfs,nfs4 -o TARGET | grep -qE '/mnt/(books|backup)'"
+    changed_when: "false"
+
+  - name: The other sixteen containers are still up
+    shell: |
+      n=$(sudo -n docker ps -q | wc -l)
+      echo "$n containers still running"
+      test "$n" -ge 10
+    changed_when: "false"