feat(nightlight): geo mode — lat/long-computed sunset via wlsunset

Set BOTH nomarchy.nightlight.latitude/.longitude and the night light
switches backing unit: wlsunset computes sunrise/sunset from the
location daily (the fixed .sunrise/.sunset are ignored;
.temperature feeds the night temp). Everything user-facing is
unchanged — same nomarchy-nightlight toggle script, Waybar moon, and
live-state ExecCondition gate; the unit name is a single let-binding
so all three follow together. Declarative-only on purpose: coordinates
are machine config like the keyboard layout, so home.nix (commented
template example), not a menu writer.

checks.nightlight-geo guards the wiring at pure eval: rendered
wlsunset ExecStart carries the coords + night temperature, the
ExecCondition gate landed on the swapped unit, hyprsunset stays off in
geo mode.

Verification: V1 — nix flake check --no-build exit 0 (evals the new
asserts + option-docs sync); built downstream-template-home (default
hyprsunset path unregressed). V3 pending: visible warm shift at the
location's night + instant toggle/off-persistence on the swapped unit
(HARDWARE-QUEUE § Any machine).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

modules/home/nightlight.nix

@@ -3,6 +3,12 @@
 # schedule (temperature/sunrise/sunset) is tuned via nomarchy.nightlight.* in
 # home.nix and baked into the unit's time-based `profile`.
 #
+# Geo mode: when BOTH nomarchy.nightlight.latitude and .longitude are set,
+# wlsunset replaces hyprsunset — it computes sunrise/sunset from the
+# location itself (the fixed .sunrise/.sunset times are ignored). Same
+# toggle script, same Waybar moon, same live-state ExecCondition gate;
+# only the unit underneath changes.
+#
 # Off by default and opt-in. Two git-tracked flags in the state file, both
 # menu-written (no ~/.local/state):
 #   settings.nightlight.installed — does the hyprsunset unit exist? Sticky; the
@@ -18,12 +24,15 @@ let
   cfg = config.nomarchy.nightlight;
   s = config.nomarchy.settings.nightlight;
   sync = lib.getExe config.nomarchy.package;
+  # Geo mode flips the backing unit; everything user-facing stays the same.
+  geo = cfg.latitude != null && cfg.longitude != null;
+  unit = if geo then "wlsunset.service" else "hyprsunset.service";
   # Runtime-on default for when the `on` key hasn't been written yet (e.g. right
   # after the first enable). Baked at eval; only used when the live key is absent.
   onDefault = lib.boolToString s.on;
 
   nomarchy-nightlight = pkgs.writeShellScriptBin "nomarchy-nightlight" ''
-    unit=hyprsunset.service
+    unit=${unit}
     # Instant runtime on/off: write the in-flake state WITHOUT a rebuild.
     write_on() { ${sync} --quiet set settings.nightlight.on "$1" --no-switch; }
     # First enable: mark the feature installed and REBUILD to create the unit
@@ -69,7 +78,7 @@ in
     # home.nix also works as a declarative opt-in.
     nomarchy.nightlight.enable = lib.mkDefault s.installed;
 
-    services.hyprsunset = lib.mkIf cfg.enable {
+    services.hyprsunset = lib.mkIf (cfg.enable && !geo) {
       enable = true;
       settings.profile = [
         # Daytime: identity = no colour change.
@@ -79,10 +88,19 @@ in
       ];
     };
 
+    # Geo mode: wlsunset owns the schedule — it recomputes sunrise/sunset
+    # from the coordinates daily (no fixed profile to bake).
+    services.wlsunset = lib.mkIf (cfg.enable && geo) {
+      enable = true;
+      latitude = cfg.latitude;
+      longitude = cfg.longitude;
+      temperature.night = cfg.temperature;
+    };
+
     # Gate the unit on the LIVE on/off state at start time (login/reboot), not
     # at eval time — so a menu toggle (written without a rebuild) is honoured on
     # the next session. A failed condition skips the unit (inactive, not failed).
-    systemd.user.services.hyprsunset.Service.ExecCondition =
+    systemd.user.services.${lib.removeSuffix ".service" unit}.Service.ExecCondition =
       lib.mkIf cfg.enable "${nomarchy-nightlight}/bin/nomarchy-nightlight should-start";
 
     home.packages = [ nomarchy-nightlight ];