Completes the remaining "Distro branding, round 2" work.
GRUB theme: hosts/live.nix builds nomarchyGrubTheme whose background is
the same composed splash as the isolinux/BIOS splash (accent logo on
base), with a palette-coloured boot menu and accent timeout bar. Derived
from nixos-grub2-theme only to reuse its bundled DejaVu .pf2; the stock
NixOS logo is dropped (ours is in the background). Wired as
isoImage.grubTheme so UEFI boot matches BIOS.
distroId: set system.nixos.distroId = "nomarchy" so os-release is honest
(ID=nomarchy, ID_LIKE=nixos). Safe — switch-to-configuration builds its
"is this NixOS?" guard from the configured distroId (and /etc/NIXOS is
the fallback), so rebuilds keep working. isNixos→false blanks the
nixos.org URLs, so extraOSReleaseArgs restores them to the project.
Adds checks.distro-id (runNixOSTest): boots distroId=nomarchy and runs
switch-to-configuration dry-activate green; os-release eval-verified from
the real downstream config; grub theme built + structure-verified.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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