feat(greeter): tuigreet + console themed from the state JSON (item 16)

New modules/nixos/greeter.nix owns the greetd block (moved out of
default.nix) plus the theming, the Plymouth model — baked at SYSTEM
rebuild from nomarchy.system.stateFile. Two parts, because tuigreet
draws on the VT with the 16 ANSI slots: console.colors becomes the
theme's ansi[] (vt.default_* kernel params — which also themes raw
ttys and the LUKS passphrase prompt), and tuigreet gets a --theme of
NAMED slots (its parser is ratatui Color::from_str, checked in source;
the names map to the standard indexes the retinted palette now
serves). A sparse state without ansi skips theming cleanly.

Verified: V0; V1 — the template-system toplevel bakes the full --theme
spec into greetd.toml and all three vt.default_{red,grn,blu} palettes
into kernel-params. A checks.greeter VM test was attempted and dropped
after bisecting: tuigreet dies under runNixOSTest even without the
flag (harness limit; nixpkgs's own greetd test uses agreety) —
recorded in agent/MEMORY.md. Remains: V3 rendering +
theme-follows-rebuild on hardware (HARDWARE-QUEUE).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

agent/MEMORY.md

@@ -7,6 +7,14 @@ here the moment a debugging session teaches you something a future
 iteration would otherwise rediscover.
 
 ## Testing & VM recipes
+- **tuigreet dies silently under runNixOSTest** (even bare, no theme
+  flag: greetd sits as "(greetd)" with no child, nothing in the
+  journal — its stderr goes to the VT) — nixpkgs' own greetd test uses
+  agreety instead. Greeter *rendering* is interactive-ISO/hardware
+  tier; don't burn another session on a checks.greeter VM test.
+- In VM tests `pgrep -f PATTERN` can match the test backdoor's own
+  `bash -c` wrapper (the pattern is in its cmdline) — use `pgrep -x`
+  or a `[t]uigreet`-style bracket pattern.
 - A checks.* fixture CANNOT be a writeText/toFile state file read at
   eval time ("path … is not valid" — flake check's eval store won't
   realise it): extract the logic into a pure importable file and