refactor(shoukei): cap battery charging from a udev rule

macsmc-battery's "add" uevent fires before the supply accepts writes
(power_supply_register bumps use_cnt after device_add); its deferred
register work then emits "change". Match both instead of polling for 30 s
from a oneshot.

Assisted-by: Claude Code (claude-opus-5-5)

hosts/12kingdoms-shoukei/apple-silicon.nix

@@ -1,5 +1,4 @@
 {
-  mylib,
   pkgs,
   nixos-apple-silicon,
   my-asahi-firmware,
@@ -66,36 +65,11 @@
     HibernateDelaySec = "5min";
   };
 
-  # Cap charging at 90%: the SMC's charge_control_end_threshold does not reliably survive a reboot,
-  # and a udev rule that writes it races the driver probe, so re-apply it from a boot-time service.
-  # TLP is not used for this: its macbook plugin still validates the 80-or-100 values of the older
-  # CHWA firmware and rejects anything else, while this machine's CHLS key takes any value. The
-  # driver ignores charge_control_start_threshold writes and derives it as end - 5.
-  systemd.services.apple-charge-limit = {
-    description = "Cap the battery charge at 90%";
-    wantedBy = [ "multi-user.target" ];
-    after = [ "systemd-modules-load.service" ];
-    path = [ pkgs.coreutils ];
-    serviceConfig = mylib.systemdHardening.privilegedOneshot // {
-      Type = "oneshot";
-      RemainAfterExit = true;
-      ExecStart = pkgs.writeShellScript "apple-charge-limit" ''
-        limit=90 # percent
-        node=/sys/class/power_supply/macsmc-battery/charge_control_end_threshold
-        # macsmc probes asynchronously, so retry briefly instead of racing the driver.
-        for _ in $(seq 1 30); do
-          if [ -e "$node" ] && echo "$limit" > "$node" 2>/dev/null &&
-            [ "$(cat "$node")" = "$limit" ]; then
-            echo "battery charge capped at $limit%"
-            exit 0
-          fi
-          sleep 1
-        done
-        echo "battery charge cap: $(cat "$node" 2>/dev/null || echo "$node missing")" >&2
-        exit 1
-      '';
-    };
-  };
+  # Cap charging at 90%. The driver rejects writes at "add" (before registration finishes) and
+  # announces readiness with "change". TLP's macbook plugin only accepts 80/100 here.
+  services.udev.extraRules = ''
+    ACTION=="add|change", SUBSYSTEM=="power_supply", KERNEL=="macsmc-battery", ATTR{charge_control_end_threshold}!="90", ATTR{charge_control_end_threshold}="90"
+  '';
 
   # After adding this snippet, you need to restart the system for the touchbar to work.
   hardware.apple.touchBar = {