feat(branding): GRUB (UEFI) theme + distroId=nomarchy

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>

flake.nix

@@ -206,6 +206,40 @@
               )
             '';
           };
+
+          # The distroId question (roadmap "Distro branding"): set
+          # distroId = "nomarchy" so /etc/os-release is honest (ID=nomarchy,
+          # ID_LIKE=nixos). The risk is switch-to-configuration's "is this
+          # NixOS?" guard — but it builds the check from the *configured*
+          # distroId (and /etc/NIXOS still exists), so a rebuild must still
+          # work. This boots such a system and runs `switch-to-configuration
+          # dry-activate` to prove the guard passes. Inline (not the full
+          # module) so the VM stays minimal — the real wiring in default.nix
+          # is covered by the downstream-template-system build.
+          distro-id = pkgs.testers.runNixOSTest {
+            name = "nomarchy-distro-id";
+            nodes.machine = { ... }: {
+              system.nixos.distroName = "Nomarchy";
+              system.nixos.distroId = "nomarchy";
+              # Test base omits switch-to-configuration by default; we need it
+              # to exercise the "is this NixOS?" guard.
+              system.switch.enable = true;
+            };
+            testScript = { nodes, ... }: ''
+              machine.wait_for_unit("multi-user.target")
+
+              # os-release reflects the rebrand, with the nixos lineage marker.
+              machine.succeed("grep -q '^ID=nomarchy$' /etc/os-release")
+              machine.succeed("grep -q '^ID_LIKE=nixos$' /etc/os-release")
+              machine.succeed("grep -q '^NAME=Nomarchy$' /etc/os-release")
+
+              # The rebuild path must still recognise this as a NixOS install
+              # (the guard is built from the configured distroId, not "nixos").
+              machine.succeed(
+                  "${nodes.machine.system.build.toplevel}/bin/switch-to-configuration dry-activate"
+              )
+            '';
+          };
         };
 
       # ─── Reference host ────────────────────────────────────────────