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.

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";