feat(tofu,ansible): SSD-backed etcd disk per k3s node (Phase 1: code + plan)

Interim bridge to stop Longhorn replica-rebuild I/O storms on the shared HDD
Storage pool from starving etcd WAL fsync (crash-looping k3s / flapping the
kube-vip VIP). Adds a small dedicated SSD disk per node for etcd only; Longhorn
stays on HDD. See docs/plans/ssd-etcd-isolation.md.

tofu (infra/tofu):
- New vars etcd_ssd_datastore (default "local-zfs" = rpool/data, Micron 5100
  SATA SSD mirror w/ PLP) and etcd_disk_gib (default 16).
- New virtio2 disk block on the SSD datastore per VM: discard=on, cache=none
  (honor guest fsync straight to the ZFS ZIL — safe on PLP SSD), iothread=false.
  Kept last (ascending interface) so bpg does not reorder virtio0/virtio1.
- Revert the debian_cloud_image resource to the long type name
  proxmox_virtual_environment_download_file to match the R2 state: bpg >= 0.111
  dropped the cross-type moved{} linkage to the short proxmox_download_file, so
  the stale moved block failed EVERY plan with "Resource type mismatch" (broken
  on main too). Aligning code to state yields a clean plan with no image
  re-download. Short-name migration is deferred to a deliberate `tofu state mv`.

ansible (node_baseline):
- Format /dev/vdc (label k3s-etcd) and mount it at
  /var/lib/rancher/k3s/server/db (defaults,noatime; no nofail so a missing etcd
  disk fails loudly rather than silently landing etcd on the root HDD) before
  k3s starts, so a rebuilt node comes up with etcd isolated on SSD. Data-safe:
  filesystem module never reformats an existing fs.

Phase 1 only: non-disruptive. No tofu apply, no node/VM/etcd/Longhorn changes.
tofu plan verified: 0 to add, 3 to change, 0 to destroy — every VM an in-place
disk ADD (virtio2), dev-3 also an in-place memory 16384->24576; no VM replace.
Merge gate green (render==golden included).

Co-Authored-By: Claude Opus 4.8 <[email protected]>

infra/tofu/variables.tf

@@ -100,6 +100,24 @@ variable "data_disk_gib" {
   default     = 128
 }
 
+# --- dedicated SSD etcd disk (interim etcd-on-SSD isolation, docs/plans/ssd-etcd-isolation.md) --
+# etcd WAL fsync was being starved by Longhorn replica-rebuild I/O on the shared HDD
+# `Storage` pool (OS + Longhorn disks). A tiny dedicated SSD disk per node moves the
+# etcd data dir (/var/lib/rancher/k3s/server/db, mounted by node_baseline) off the HDD.
+# Interim bridge until the all-SSD mini-PCs land; Longhorn stays on HDD (does not fit).
+
+variable "etcd_ssd_datastore" {
+  description = "SSD-backed datastore for the dedicated etcd disk (VERIFIED: local-zfs = rpool/data, ZFS mirror on Micron 5100 SATA SSDs w/ power-loss protection)."
+  type        = string
+  default     = "local-zfs"
+}
+
+variable "etcd_disk_gib" {
+  description = "Dedicated SSD etcd disk per node, GiB (etcd DB is ~83 MB; 16 GiB is generous headroom)."
+  type        = number
+  default     = 16
+}
+
 variable "cpu_type" {
   description = "Proxmox vCPU type. `host` passes through the physical CPU (nested-virt for KubeVirt, Phase 5)."
   type        = string