docs(esh): record the 2-node cluster, and that a dark tile is pvestatd

Two findings from chasing node `pve` showing dark in the UI.

esh-pve and esh-pve-nas are a 2-node cluster (esh-pve-cluster, expected
votes 2, quorum 2, no qdevice). This was undocumented, and last night's
migration rebooted one of the members without accounting for it. Nothing
broke -- quorum is intact and both nodes report the same ring id -- but
that was luck. Rebooting either node drops the survivor below quorum and
makes its /etc/pve read-only until the partner returns. It matters for
the pending confirmation reboot and the 225-package upgrade, both of
which take a node down.

The dark tile itself was NOT last night's doing. pvestatd SEGV'd on
2026-05-28 and had been dead 82 days; the journal has nothing between
that crash and the restart today. It is only the reporting daemon, so
the node stayed quorate and healthy with all services active and all
three guests running the whole time -- the UI simply had nothing telling
it the node was alive. Fourth SEGV in that unit's history, so treat a
recurrence as expected and consider a watchdog: nothing alerts on it,
and the sole symptom is cosmetic enough to go unnoticed for months.

servers/esh-pve/README.md

@@ -33,3 +33,25 @@ Same Proxmox-inspect caveat as `pfi-pve`.
 ## Placement rule
 
 Hypervisor for ESH home-lab VMs. Not part of the PFI colo topology.
+
+## ⚠ Cluster membership and the dark-tile gotcha
+
+This node (`pve`) is half of the 2-node **`esh-pve-cluster`** with `esh-nas-pve`
+(esh-pve-nas, 10.0.50.55). `Expected votes: 2`, `Quorum: 2`, no qdevice — so
+**rebooting either node makes the survivor's `/etc/pve` read-only** until the
+partner is back. Guests keep running; config changes do not. Plan reboots of
+either node accordingly (`pvecm expected 1` on the survivor, or accept the
+read-only window).
+
+⚠ **A dark/greyed node tile usually means `pvestatd`, not a dead node.**
+`pvestatd` SEGV'd here on **2026-05-28 and stayed dead for 82 days** — the node
+was quorate and healthy the entire time, with `pve-cluster`, `corosync`,
+`pvedaemon`, `pveproxy` and both HA services active and all three guests running.
+It is only the *reporting* daemon, so its death is invisible except that the UI
+has nothing to render. It has SEGV'd four times (2025-08-18, 2025-09-04,
+2025-09-15, 2026-05-28) — treat a recurrence as expected, not novel.
+
+    systemctl reset-failed pvestatd && systemctl restart pvestatd
+
+Restarted 2026-08-18. Worth a watchdog: nothing alerts when it dies, and the only
+symptom is a cosmetic one nobody looks at for months.