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>

pkgs/nomarchy-theme-sync/nomarchy-theme-sync.py

@@ -165,12 +165,12 @@ def run_switch() -> None:
     # Persistent (timeout 0): stays up for the whole rebuild — replaced
     # in place by the success/failure notification below — so a multi-
     # minute switch never looks like it silently failed.
-    notify("Applying theme — rebuilding the desktop…", persistent=True)
+    notify("Applying changes — rebuilding the desktop…", persistent=True)
     result = subprocess.run(argv)  # stream output to the caller's terminal
     if result.returncode != 0:
-        notify("Theme rebuild FAILED — see terminal / journal", urgency="critical")
+        notify("Rebuild FAILED — see terminal / journal", urgency="critical")
         die("rebuild failed (state file already updated; fix and re-run)")
-    notify("Theme applied ✓")
+    notify("Changes applied ✓")
     # Waybar runs from Hyprland's exec-once (not a systemd unit HM would restart
     # on switch), so nudge the running bar to re-read its freshly rebuilt
     # config/style. SIGUSR2 = reload; harmless no-op if waybar isn't running.
@@ -294,7 +294,11 @@ def cmd_set(args) -> None:
         check_fonts(state)
     if not args.no_switch:
         run_switch()
-        apply_wallpaper(state)
+        # Only wallpaper/theme keys change what swww shows; skip the re-apply
+        # (and its transition) for unrelated keys like ui.* or settings.* so a
+        # gaps tweak or a feature toggle doesn't flash the background.
+        if args.path.split(".")[0] in ("wallpaper", "slug"):
+            apply_wallpaper(state)
 
 
 def cmd_get(args) -> None: