test-lab: add two-node HA file-server test cluster config

Disposable test VMs (ha-test-node1 / ha-test-node2, VMIDs 200/201 on pve1)
to evaluate whether the DRBD + XFS + LIO + Corosync + Pacemaker stack
runs correctly on NixOS before deciding NixOS vs Debian for production.

Includes:
- test-lab/ha/disko.nix: 20G boot disk layout (smaller than production)
- test-lab/ha/common.nix: shared HA stack (drbd, corosync, pacemaker,
  targetcli-fb, xfsprogs), OCF PATH workaround for nixpkgs#207891
- test-lab/ha/node1.nix / node2.nix: per-node hostname + static IP
- test-lab/ha/fence-pve-ssh.py: Proxmox SSH fence agent for STONITH
- test-lab/ha/cluster-init.sh: one-shot cluster bootstrap script
- test-lab/ha/cluster-enable-stonith.sh: enables STONITH post-key-deploy
- flake.nix: adds ha-test-node1 / ha-test-node2 nixosConfigurations
  (bypasses mkTarget / clan-core / sops-nix — test-only)

These VMs must be destroyed once acceptance testing is complete.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

test-lab/ha/disko.nix

@@ -0,0 +1,49 @@
+{ config, ... }:
+
+# Smaller disk layout for throwaway test VMs (20G vs production 50G).
+# Same partition scheme as modules/disko/proxmox.nix: GPT, ESP + swap + ext4 root.
+# Only covers the boot disk (scsi0 → /dev/sda).  The DRBD data disk
+# (scsi1 → /dev/sdb) is left raw — drbdadm create-md initialises it.
+{
+  disko.devices.disk.main = {
+    type = "disk";
+    device = "/dev/sda";
+    imageSize = "20G";
+    imageName = config.networking.hostName;
+
+    content = {
+      type = "gpt";
+      partitions = {
+        esp = {
+          priority = 1;
+          name = "ESP";
+          size = "512M";
+          type = "EF00";
+          content = {
+            type = "filesystem";
+            format = "vfat";
+            mountpoint = "/boot";
+            mountOptions = [ "umask=0077" ];
+            extraArgs = [ "-F" "32" "-n" "boot" ];
+          };
+        };
+        swap = {
+          size = "2G";
+          content = {
+            type = "swap";
+            randomEncryption = false;
+          };
+        };
+        root = {
+          size = "100%";
+          content = {
+            type = "filesystem";
+            format = "ext4";
+            mountpoint = "/";
+            extraArgs = [ "-L" "nixos" ];
+          };
+        };
+      };
+    };
+  };
+}