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>

.claude/settings.local.json

@@ -273,7 +273,10 @@
       "Bash(nix-instantiate --parse modules/nixos/plymouth.nix)",
       "Bash(echo \"parse OK\")",
       "Bash(echo \"=== build the plymouth theme derivation directly ===\")",
-      "Bash(nix-instantiate --parse modules/nixos/default.nix)"
+      "Bash(nix-instantiate --parse modules/nixos/default.nix)",
+      "Bash(cp /nix/store/f18ij42lmpg07fn1m29aplayakx7kqpz-splash-test.png/* preview-splash-built.png)",
+      "Bash(cp /nix/store/f18ij42lmpg07fn1m29aplayakx7kqpz-splash-test.png preview-splash-built.png)",
+      "Bash(nix-instantiate --parse hosts/live.nix)"
     ]
   }
 }