feat(branding): ISO boot splash from the vendored Nomarchy logo

Vendor the vector logo from the legacy branch
(modules/nixos/branding/logo.svg) and use it for the live ISO's isolinux
splash: rsvg-convert renders it, recolored to the palette accent and
centred on the theme base (from theme-state.json) at build time, wired via
isoImage.splashImage in hosts/live.nix. Verified the generated PNG by
building it in the sandbox.

UEFI/GRUB still uses the stock theme (grubTheme needs a full theme dir —
left on the roadmap), as does the distroId switch.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

README.md

@@ -318,8 +318,11 @@ close · `SUPER+1..9` workspaces · `Print` region screenshot.
   (os-release `PRETTY_NAME`, systemd-boot entries, ISO menu label).
   ✓ tuigreet greeting (`Welcome to <distroName>`) and a branded `users.motd`
   (doubling as a helper cheat sheet), both keyed off `distroName`.
-  Remaining: `isoImage.splashImage`/`grubTheme` art for the ISO boot
-  screen (needs a designed asset), and the `distroId` question (it changes
+  ✓ `isoImage.splashImage` — the vendored vector logo
+  (`modules/nixos/branding/logo.svg`, from legacy) recolored to the palette
+  accent on the theme base, built at ISO-build time (`hosts/live.nix`).
+  Remaining: `isoImage.grubTheme` so UEFI boot matches the isolinux splash
+  (needs a full grub theme dir), and the `distroId` question (it changes
   `DEFAULT_HOSTNAME` and upstream `isNixos` checks — needs a test pass;
   nixos-* CLI names stay regardless)
 - **Theme-switch feedback:** ✓ the "rebuilding…" notification is now