refactor(#107): theme-state.json → state.json, theme-sync → state-sync

The machine flake's git-tracked settings file is system state, not
"theme" only — rename it to state.json. CLI becomes nomarchy-state-sync
with a nomarchy-theme-sync symlink for scripts and muscle memory.

Eval (mkFlake, doctor, lifecycle) still accepts theme-state.json; the
next write migrates to state.json and removes the legacy file.
Documented in MIGRATION.md; drop the CLI alias after release notes.

modules/nixos/plymouth.nix

@@ -1,10 +1,10 @@
 # Plymouth boot splash — Nomarchy-branded (ported from the legacy
-# iteration), background tinted from the same theme-state.json that
+# iteration), background tinted from the same state.json that
 # drives the desktop (nomarchy.system.stateFile, wired automatically by
 # lib.mkFlake). One caveat by design: theme switches are Home
 # Manager-only and never touch the initrd, so the splash follows the
 # theme as of the last SYSTEM rebuild (`sys-update`), not the last
-# `nomarchy-theme-sync apply`.
+# `nomarchy-state-sync apply`.
 { config, lib, pkgs, ... }:
 
 let
@@ -12,7 +12,7 @@ let
 
   state =
     if cfg.stateFile != null
-    then import ../theme-state-read.nix { inherit lib; } cfg.stateFile
+    then import ../state-read.nix { inherit lib; } cfg.stateFile
     else { };
   colorOf = key: fallback: lib.removePrefix "#" ((state.colors or { }).${key} or fallback);
   # Fallbacks match the distro default theme (Boreal) when stateFile is null.