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>

hosts/live.nix

@@ -4,26 +4,85 @@
 { lib, pkgs, username, nomarchySrc, ... }:
 
 let
-  # ISO boot splash: the Nomarchy monogram recolored to the palette accent,
+  # ISO boot branding: the Nomarchy monogram recolored to the palette accent,
   # centred on the theme base. Built from the vendored vector logo and the
-  # live theme-state.json (tokyo-night by default). Shows on the isolinux
-  # (BIOS) boot menu; UEFI/GRUB still uses the stock theme (see roadmap).
+  # live theme-state.json (tokyo-night by default). The same composed image
+  # backs both the isolinux (BIOS) splash and the GRUB (UEFI) theme below, so
+  # the two boot paths match.
   state = builtins.fromJSON (builtins.readFile ../theme-state.json);
   isoColor = key: fallback: lib.removePrefix "#" ((state.colors or { }).${key} or fallback);
+  accent = isoColor "accent" "7aa2f7";
+  base = isoColor "base" "1a1b26";
+  subtext = isoColor "subtext" "787c99";
+
   isoSplash = pkgs.runCommand "nomarchy-iso-splash.png"
     { nativeBuildInputs = [ pkgs.imagemagick pkgs.librsvg ]; } ''
     rsvg-convert -h 320 ${../modules/nixos/branding/logo.svg} > logo.png
-    magick logo.png -fill "#${isoColor "accent" "7aa2f7"}" -colorize 100 logo-c.png
-    magick -size 1920x1080 xc:"#${isoColor "base" "1a1b26"}" \
+    magick logo.png -fill "#${accent}" -colorize 100 logo-c.png
+    magick -size 1920x1080 xc:"#${base}" \
       logo-c.png -gravity center -composite $out
   '';
+
+  # GRUB (UEFI) theme matched to the BIOS splash: the same accent-logo-on-base
+  # image as the background, plus a palette-coloured boot menu in the lower
+  # third (clear of the centred logo). Derived from nixos-grub2-theme only to
+  # reuse its bundled DejaVu .pf2 font — we overwrite the background and
+  # theme.txt and drop the stock NixOS logo (ours is in the background). grub
+  # loads every .pf2 in the dir, so "DejaVu Regular" resolves.
+  grubThemeTxt = pkgs.writeText "nomarchy-grub-theme.txt" ''
+    title-text: ""
+    desktop-image: "background.png"
+    desktop-color: "#${base}"
+
+    message-font: "DejaVu Regular"
+    message-color: "#${subtext}"
+    terminal-font: "DejaVu Regular"
+
+    + boot_menu {
+      left = 50%-300
+      width = 600
+      top = 64%
+      height = 26%
+      item_font = "DejaVu Regular"
+      item_color = "#${subtext}"
+      item_height = 36
+      item_spacing = 6
+      selected_item_font = "DejaVu Regular"
+      selected_item_color = "#${accent}"
+      scrollbar = false
+    }
+
+    + progress_bar {
+      id = "__timeout__"
+      left = 50%-300
+      top = 92%
+      width = 600
+      height = 16
+      show_text = true
+      text = "@TIMEOUT_NOTIFICATION_MIDDLE@"
+      font = "DejaVu Regular"
+      text_color = "#${subtext}"
+      border_color = "#${accent}"
+      bg_color = "#${base}"
+      fg_color = "#${accent}"
+    }
+  '';
+
+  nomarchyGrubTheme = pkgs.runCommand "nomarchy-grub-theme" { } ''
+    cp -r ${pkgs.nixos-grub2-theme} $out
+    chmod -R u+w $out
+    cp ${isoSplash} $out/background.png
+    cp ${grubThemeTxt} $out/theme.txt
+    rm -f $out/logo.png
+  '';
 in
 {
   networking.hostName = "nomarchy-live";
 
   isoImage.volumeID = lib.mkForce "NOMARCHY_LIVE";
   isoImage.edition = lib.mkForce "live";
-  isoImage.splashImage = isoSplash;
+  isoImage.splashImage = isoSplash; # isolinux / BIOS
+  isoImage.grubTheme = nomarchyGrubTheme; # GRUB / UEFI
 
   # The minimal-CD profile slims the image for a CONSOLE installer; this
   # ISO is the desktop, so re-enable what it strips. Above all