feat(autotheme): fire at the configured sunrise/sunset, not a 15-min poll

The timer's OnCalendar now bakes the configured times (falling back
like _hhmm_to_min on a hand-edited state), so the switch lands at the
configured minute instead of up to 15 minutes late. The original
poll's robustness case is covered without it: Persistent catches
transitions missed while powered off, systemd fires elapsed calendar
timers on resume, and OnStartupSec settles the theme after login.

Baked times mean a time edit must rebuild: the menu's Sunrise/Sunset
writes now run `auto --force` when enabled (one rebuild re-bakes the
timer and lands on the right theme, same as the enable flow). Day and
night slugs stay live reads — but with no poll to catch them up,
editing the slot currently on screen applies immediately via a plain
`auto`. New eval check `auto-theme-timer` asserts the bake
state-bridges-style: configured times reach OnCalendar, garbage falls
back to 07:00/20:00, disabled installs no unit. Docs + hardware-queue
entry updated to the new semantics.

V2: checks.auto-theme (KVM VM, passed) covers the auto engine the
timer ticks; checks.auto-theme-timer (eval, passed) covers the unit;
OnCalendar strings systemd-analyze-validated; the rebuilt menu script
passes bash -n with the new branches in place. V3 pending: live timer
firing at the exact configured minute on hardware (existing
"Auto-theme pair flip" queue entry, refreshed).

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

modules/home/autotheme.nix

@@ -1,18 +1,37 @@
 # Auto time-of-day theme switch (BACKLOG #79, VISION § D). A user timer
-# periodically runs `nomarchy-state-sync auto`, which reads
-# settings.autoTheme = { enable, day, night, sunrise, sunset } from the
-# state file and applies the day or night preset for the current clock —
-# through the SAME one engine as a manual `apply` (no second pipeline).
+# runs `nomarchy-state-sync auto` at the configured sunrise and sunset,
+# which reads settings.autoTheme = { enable, day, night, sunrise, sunset }
+# from the state file and applies the day or night preset for the current
+# clock — through the SAME one engine as a manual `apply` (no second
+# pipeline).
 #
-# The command self-gates (no-op when disabled) and is idempotent (it only
-# rebuilds when the active theme actually needs to change), so the timer
-# can tick freely. Install is gated on the state flag (like nomarchy.updates
-# gates on its enable), so a machine not using the feature carries no timer;
-# enabling it from the menu (slice 3) writes the flag + rebuilds, which is
-# what brings the timer into being.
+# The trigger times are BAKED into OnCalendar at rebuild (originally this
+# was a 15-min poll; exact times won on wasted wakeups — Bernardo
+# 2026-07-18), so a time edit must rebuild: the menu's Sunrise/Sunset
+# writes run `auto --force`, whose apply is that rebuild. Missed
+# transitions are covered without polling: Persistent=true catches ones
+# that pass while powered off, systemd fires elapsed OnCalendar timers on
+# resume from suspend, and OnStartupSec settles the theme just after
+# login. The command self-gates (no-op when disabled) and is idempotent
+# (it only rebuilds when the active theme actually needs to change), so
+# every extra firing is cheap.
+#
+# Install is gated on the state flag (like nomarchy.updates gates on its
+# enable), so a machine not using the feature carries no timer; enabling
+# it from the menu (slice 3) writes the flag + rebuilds, which is what
+# brings the timer into being.
 { config, lib, pkgs, ... }:
 
-lib.mkIf (config.nomarchy.settings.autoTheme.enable or false) {
+let
+  at = config.nomarchy.settings.autoTheme or { };
+  # "HH:MM" or the fallback — mirrors _hhmm_to_min in nomarchy-state-sync,
+  # so a hand-edited state can't bake an OnCalendar systemd rejects.
+  hhmm = v: fallback:
+    if builtins.isString v
+       && builtins.match "([01][0-9]|2[0-3]):[0-5][0-9]" v != null
+    then v else fallback;
+in
+lib.mkIf (at.enable or false) {
   systemd.user.services.nomarchy-auto-theme = {
     Unit.Description = "Apply the day/night theme for the current time";
     Service = {
@@ -25,10 +44,13 @@ lib.mkIf (config.nomarchy.settings.autoTheme.enable or false) {
   };
 
   systemd.user.timers.nomarchy-auto-theme = {
-    Unit.Description = "Periodic day/night theme check";
+    Unit.Description = "Day/night theme switch at sunrise and sunset";
     Timer = {
       OnStartupSec = "1min";   # settle on the right theme shortly after login
-      OnCalendar = "*:0/15";   # re-check every 15 min; auto no-ops if unchanged
+      OnCalendar = [
+        "*-*-* ${hhmm (at.sunrise or null) "07:00"}:00"  # sunrise → day
+        "*-*-* ${hhmm (at.sunset  or null) "20:00"}:00"  # sunset  → night
+      ];
       Persistent = true;       # catch a transition missed while powered off
     };
     Install.WantedBy = [ "timers.target" ];