Stage a ZFS RAID0 disko layout for the bare-metal gui host

Adds modules/disko/baremetal.nix: two disks, each its own top-level
zpool vdev with no mirror/raidz between them (disko's zpool `mode`
defaults to "" for a plain stripe), ESP + systemd-boot on disk1. Device
paths are placeholders in variables.nix (guiRootDisk1/guiRootDisk2)
until the real hardware profile arrives.

Verified structurally by building a throwaway nixosSystem with the
actual disko.nixosModules.disko and reading the generated
system.build.formatScript: it emits `zpool create rpool ... disk1
disk2` with no mirror/raidz keyword, confirming a genuine stripe.

Not yet wired into any flake target -- that happens once the hardware
config lands and a new bare-metal platform module is added, per the
agreed sequencing.

modules/disko/baremetal.nix

@@ -0,0 +1,87 @@
+{ vars, ... }:
+
+{
+  # ZFS RAID0 (striped, no redundancy) root pool for the bare-metal gui
+  # host — two disks, each contributing its own top-level vdev. disko's
+  # zpool `mode` defaults to "" (plain stripe) when left unset, which is
+  # what gives RAID0 semantics here rather than mirror/raidz.
+  #
+  # Device paths are placeholders until the real hardware profile lands —
+  # fill in vars.guiRootDisk1/guiRootDisk2 (stable /dev/disk/by-id/...
+  # paths, not /dev/sdX) before running disko against real hardware. Swap
+  # is deliberately left out for now — sizing that sensibly needs the
+  # box's actual RAM size, which comes with the hardware profile too.
+  #
+  # Not yet imported anywhere: this awaits the new bare-metal platform
+  # module (alongside modules/boot/efi.nix for systemd-boot, matching
+  # modules/platforms/proxmox.nix's pattern) once the hardware config is
+  # in hand.
+  disko.devices = {
+    disk = {
+      disk1 = {
+        type = "disk";
+        device = vars.guiRootDisk1;
+
+        content = {
+          type = "gpt";
+
+          partitions = {
+            esp = {
+              priority = 1;
+              name = "ESP";
+              size = "512M";
+              type = "EF00";
+
+              content = {
+                type = "filesystem";
+                format = "vfat";
+                mountpoint = "/boot";
+                mountOptions = [ "umask=0077" ];
+              };
+            };
+
+            zfs = {
+              size = "100%";
+
+              content = {
+                type = "zfs";
+                pool = "rpool";
+              };
+            };
+          };
+        };
+      };
+
+      disk2 = {
+        type = "disk";
+        device = vars.guiRootDisk2;
+
+        content = {
+          type = "gpt";
+
+          partitions = {
+            zfs = {
+              size = "100%";
+
+              content = {
+                type = "zfs";
+                pool = "rpool";
+              };
+            };
+          };
+        };
+      };
+    };
+
+    zpool.rpool = {
+      type = "zpool";
+
+      rootFsOptions = {
+        compression = "zstd";
+        "com.sun:auto-snapshot" = "false";
+      };
+      mountpoint = "/";
+      options.ashift = "12";
+    };
+  };
+}

variables.nix

@@ -52,6 +52,12 @@
   wifiSsid = "";
   wifiPassword = "";
 
+  # Bare-metal gui host's two disks for a ZFS RAID0 (striped) root pool
+  # (modules/disko/baremetal.nix). Use stable /dev/disk/by-id/... paths,
+  # not /dev/sdX -- fill in once the hardware profile is available.
+  guiRootDisk1 = "";
+  guiRootDisk2 = "";
+
   # System
   timeZone = "Australia/Brisbane";