feat(nightlight): menu-driven, in-flake on/off (no ~/.local/state)

Night light is off by default and configured entirely through the menu,
writing into the downstream flake instead of ~/.local/state — so the
choice is git-tracked and reproducible across a clone+rebuild.

Two flags under a new settings.* section of the state file (exposed as
the nomarchy.settings option):
- settings.nightlight.installed — sticky, gates the hyprsunset unit;
  nomarchy.nightlight.enable mkDefault-reads it. First enable from the
  menu rebuilds to create the unit (the one accepted rebuild).
- settings.nightlight.on — runtime on/off, toggled instantly
  (theme-sync set --no-switch + systemctl), no rebuild. An ExecCondition
  reads the live flag at session start so an off survives reboot;
  splitting it from the sticky flag means a later unrelated rebuild
  never undoes an instant-off (no decay).

Drops the ~/.local/state/nomarchy/nightlight-off marker and the
ConditionPathExists hack. theme-sync set now skips the wallpaper
re-apply for non-wallpaper keys, and its rebuild notifications are
generic ("changes" not "theme").

First cut of the broader principle — any user-settable config gets a
menu writer that lands it in the flake; no state outside the checkout.
Docs (README §4, ROADMAP, template) updated to reflect it.

Eval- and round-trip-verified; on-hardware check pending.

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

modules/home/options.nix

@@ -255,6 +255,17 @@ in
       description = "The parsed theme state (stateFile merged over defaults).";
     };
 
+    settings = lib.mkOption {
+      type = lib.types.attrs;
+      readOnly = true;
+      description = ''
+        Parsed feature settings — the `settings` section of the state file,
+        what the menu/Waybar toggles write (e.g. settings.nightlight.enable).
+        Feature options mkDefault-read from here, so a menu toggle lands in the
+        in-flake state (git-tracked, reproducible) rather than ~/.local/state.
+      '';
+    };
+
     lib = lib.mkOption {
       type = lib.types.attrs;
       readOnly = true;