fix(nixos): two toggles that reported success and did nothing — wire the state bridges

BACKLOG #116. `nomarchy.settings` is declared in exactly one place —
modules/home/options.nix:412, the Home Manager side. On NixOS the attribute
does not exist, and `or <fallback>` swallows the missing-attribute error, so
four options that "defaulted from the state" had silently been their fallback
on every machine ever built.

The item said three options, and called them benign. Both halves were wrong,
and re-grepping rather than trusting the account is what found it:

  * There were four. The original enumeration read options.nix instead of
    modules/nixos/ and missed services.nix's printing.enable — the same
    mistake in miniature as the bug it was filing.
  * Two were live user-facing bugs. Control Center is shipped
    (default.nix:337) and reachable from the menu; its Bluetooth and Printing
    toggles wrote settings.{bluetooth,printing}.enable and printed "requires
    rebuild", and the rebuild changed nothing. They had never worked.

The fix is one shape, now uniform: the option declares a STATIC default, and
the implementing module reads the state via theme-state-read.nix (fails closed
on bad JSON, unlike greeter.nix's raw fromJSON — also moved onto the reader
here) and mkDefaults it behind `mkIf (state != null)`. An absent key leaves the
option default as the single source of the fallback; a hand-set system.nix
value still pins it. batteryChargeLimit gets no bridge and loses its dead read:
power.nix's oneshot already reads that key with jq at RUNTIME and prefers it
over the baked value, which is why that menu worked all along.

V2. The bug is proved real before/after on the same flipped state: BEFORE,
bluetooth stays true and printing stays false; AFTER, both flip, and a hand-set
value still outranks the state. Nothing in a build fails when a bridge dies, so
the guards are the point — checks.state-bridges asserts 11 eval cases, and
checks.printing-from-state boots a VM whose only input is the state file and
waits for a running cups.service. The guard was itself proved to fail:
re-breaking the bluetooth bridge makes it throw, naming both assertions. A
check that passes whether or not the property holds is worse than no check
(625b7e3). flake check, option-docs, template-sot, downstream-template-*,
installer-safety, hardware-toggles and battery-charge-limit all pass.

No V3: the mechanism is fully proved headlessly. Design record in ROADMAP §
NixOS-side state bridges (#116); new #117 (PROPOSED) for the control-center
toggles still leaving the rebuild to the user.

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

modules/nixos/default.nix

@@ -20,6 +20,19 @@ let
   nomarchyLogoFont = pkgs.runCommandLocal "nomarchy-logo-font" { } ''
     install -Dm444 ${./branding/Nomarchy.ttf} $out/share/fonts/truetype/Nomarchy.ttf
   '';
+
+  # Control Center's Bluetooth toggle writes settings.bluetooth.enable; read
+  # it from the state file, the only place it exists on the NixOS side (the
+  # hardware.nix/timezone.nix bridge). Missing/invalid JSON fails closed via
+  # theme-state-read.nix rather than a raw stack. null = key absent, which
+  # leaves the option's own default (true) alone.
+  sysState =
+    if cfg.stateFile != null
+    then import ../theme-state-read.nix { inherit lib; } cfg.stateFile
+    else { };
+  stateBluetooth =
+    let v = (sysState.settings or { }).bluetooth.enable or null;
+    in if builtins.isBool v then v else null;
 in
 {
   imports = [ ./options.nix ./plymouth.nix ./greeter.nix ./file-manager.nix ./power.nix ./services.nix ./hardware.nix ./timezone.nix ./oom.nix ];
@@ -217,6 +230,13 @@ in
     # overrides this regardless.
     users.defaultUserShell = lib.mkOverride 500 pkgs.zsh;
 
+    # The in-flake state drives the toggle; mkDefault so a hand-set
+    # nomarchy.system.bluetooth.enable in system.nix still pins it (the
+    # greeter.autoLogin shape). mkIf, not a fallback expression, so an absent
+    # key leaves the option default as the single source of `true`.
+    nomarchy.system.bluetooth.enable =
+      lib.mkIf (stateBluetooth != null) (lib.mkDefault stateBluetooth);
+
     hardware.bluetooth.enable = lib.mkDefault cfg.bluetooth.enable;
     services.blueman.enable = lib.mkDefault cfg.bluetooth.enable;