fix: centralize state defaults via lib/state-schema.nix

Kills a recurring bug class: state defaults previously lived in three
parallel places that drifted apart over time.

  - lib/state-schema.nix          (the canonical schema, referenced
                                   nowhere except a description string)
  - core/system/options.nix       (default = "..." clauses on options)
  - core/home/options.nix         (same, on home options)
  - core/home/state.nix           (`or "..."` fallbacks for state.json reads)

When `state.json` is missing a key, three files have to agree on the
fallback. They keep silently drifting:

  - The OOTB QA audit shipped fixes for this pattern.
  - Earlier this session, `chore: switch default theme summer-night → nord`
    fixed core/system/options.nix and core/home/state.nix — but missed
    core/home/options.nix, which still defaulted nomarchy.theme to
    "summer-night". Every consumer of the home option
    (features/default.nix, vscode.nix, waybar, hyprland, theme engine)
    resolved to the wrong theme when state.json was blank.

This change:

  - Imports lib/state-schema.nix into all three consumers and replaces
    every hardcoded default with `schema.<scope>.<key>`.
  - Fixes the lingering nomarchy.theme = "summer-night" home-side bug as
    a side-effect.
  - Touches roughly 25 literals across the three files.

Verified `nix flake check --no-build` passes and every centralized value
evaluates to the exact literal it previously had. Off-schema option-only
defaults (isLightMode, formFactor, cursor.*, iconsTheme, keyring.enable,
etc.) are left hardcoded — they have no state.json counterpart, so
there's no source-of-truth split to resolve.

Out of scope (follow-up):
  - Have installer/install.sh generate /mnt/etc/nixos/state.json from
    the schema instead of hardcoded JSON — would close the last
    split-brain surface (the installer can still drift from schema).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

core/system/options.nix

@@ -1,27 +1,32 @@
 { config, lib, pkgs, ... }:
 
+let
+  # Defaults live in lib/state-schema.nix so they can't drift between this
+  # file, core/home/options.nix, and core/home/state.nix's `or` fallbacks.
+  schema = import ../../lib/state-schema.nix { inherit lib; };
+in
 {
   options.nomarchy.system = {
     dns = lib.mkOption {
       type = lib.types.enum [ "Cloudflare" "Google" "DHCP" "Custom" ];
-      default = "DHCP";
+      default = schema.system.dns;
       description = "Selected DNS provider.";
     };
     customDns = lib.mkOption {
       type = lib.types.listOf lib.types.str;
-      default = [];
+      default = schema.system.customDns;
       description = "List of custom DNS servers.";
     };
     wifi = {
       powersave = lib.mkOption {
         type = lib.types.bool;
-        default = true;
+        default = schema.system.wifi.powersave;
         description = "Whether to enable wifi power saving.";
       };
     };
     timezone = lib.mkOption {
       type = lib.types.str;
-      default = "UTC";
+      default = schema.system.timezone;
       description = "System timezone.";
     };
     formFactor = lib.mkOption {
@@ -39,24 +44,24 @@
     features = {
       fingerprint = lib.mkOption {
         type = lib.types.bool;
-        default = false;
+        default = schema.system.features.fingerprint;
         description = "Whether to enable fingerprint support.";
       };
       fido2 = lib.mkOption {
         type = lib.types.bool;
-        default = false;
+        default = schema.system.features.fido2;
         description = "Whether to enable FIDO2 support.";
       };
       hybridGPU = lib.mkOption {
         type = lib.types.bool;
-        default = false;
+        default = schema.system.features.hybridGPU;
         description = "Whether to enable hybrid GPU support (supergfxd).";
       };
     };
     theme = lib.mkOption {
       type = lib.types.str;
-      default = "nord";
-      description = "Selected system theme. Matches lib/state-schema.nix and the installer-written state.json so a missing/blank state file lands on the same theme everywhere.";
+      default = schema.system.theme;
+      description = "Selected system theme.";
     };
 
     # ----- Tier 1 system features (all opt-in, no behavioural change off) ---