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.
