feat(power): re-apply battery charge limit on AC state change

The threshold was written boot-only, so a firmware that clears
charge_control_end_threshold on unplug would silently lose the cap
until the next reboot. Add a services.udev rule on the mains adapter
(ATTR{type}=="Mains", vendor-neutral) that restarts the oneshot via
systemctl --no-block on every AC state change. Uses restart (not
try-restart) so it re-applies even if the boot run was inactive.

Eval-verified: rule present when the charge limit is on, absent when
off; downstream-template-system still evaluates.

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

docs/ROADMAP.md

@@ -347,7 +347,17 @@ how to override it. Items marked ✓ are shipped.
     turns it on for a `GenuineIntel` CPU.
   - ✓ **longevity:** `power.batteryChargeLimit` (e.g. 80) — a backend-
     independent sysfs oneshot writes `charge_control_end_threshold`. Off by
-    default; the installer scaffolds it commented-out on laptops.
+    default; the installer scaffolds it commented-out on laptops. ✓ the
+    threshold is now **re-applied on AC state changes** (a `services.udev`
+    rule on `SUBSYSTEM=="power_supply", ATTR{type}=="Mains"` restarts the
+    oneshot via `systemctl --no-block`), closing the firmware-resets-on-
+    unplug gap — the match is by adapter *type*, not kernel name
+    (AC/AC0/ADP1/ACAD vary), and `restart` (not `try-restart`) re-applies
+    even if the boot run was inactive. Eval-verified both ways (rule present
+    with charge limit on / absent off). **Pending an on-hardware check** (a
+    `sudo nixos-rebuild` + a physical unplug to fire the udev event — the
+    dev box has both an `AC` Mains adapter and a `BAT0`
+    `charge_control_end_threshold`, so it can exercise it).
   - ✓ **installer:** writes `power.laptop = true` (battery probe) and
     `power.thermal.enable` (Intel) into the generated `system.nix`.
   - ✓ **idle cohesion:** `modules/home/idle.nix` now suspends only on
@@ -357,8 +367,6 @@ how to override it. Items marked ✓ are shipped.
     logind's lid handling is left at its defaults (suspend on lid close,
     ignore when docked) — an explicit "I'm done" that coheres with the
     idle behaviour.
-  - Remaining: a boot-only→event-driven charge-limit re-apply (udev) if a
-    firmware resets the threshold on unplug.
   - ✓ **Waybar parity:** the `custom/powerprofile` indicator now shows in the
     summer-day/night whole-swap themes too. `powerProfileStatus`/`Cycle` are
     named `writeShellScriptBin`s on PATH (`waybar.nix` home.packages), so the

modules/nixos/options.nix

@@ -109,8 +109,9 @@
           where the hardware exposes a charge threshold
           (/sys/class/power_supply/BAT*/charge_control_end_threshold).
           null leaves charging at the firmware default. Backend-independent
-          (a small systemd unit writes the sysfs knob at boot), so it
-          works under PPD too; needs nomarchy.system.power.laptop.
+          (a small systemd unit writes the sysfs knob, re-applied on AC
+          state changes), so it works under PPD too; needs
+          nomarchy.system.power.laptop.
         '';
       };
     };

modules/nixos/power.nix

@@ -38,10 +38,10 @@ in
 
     # Battery charge limit via sysfs. PPD can't cap charge at all, and
     # TLP's own knob only applies under TLP — so a tiny oneshot writes
-    # the threshold directly, independent of the backend. Boot-time only:
-    # some firmwares reset the threshold on unplug; revisit with a udev
-    # hook if that bites. `-` paths that don't exist are skipped, so this
-    # is a clean no-op on hardware without the control.
+    # the threshold directly, independent of the backend. Re-applied on
+    # AC state changes by the udev rule below (some firmwares reset the
+    # threshold when the charger is unplugged). `[ -w ]` skips paths that
+    # don't exist, so this is a clean no-op on hardware without the control.
     systemd.services.nomarchy-battery-charge-limit = lib.mkIf chargeLimit {
       description = "Cap battery charging at ${toString cfg.batteryChargeLimit}%";
       wantedBy = [ "multi-user.target" ];
@@ -56,5 +56,16 @@ in
         exit 0
       '';
     };
+
+    # Re-apply the threshold whenever the mains adapter changes state: the
+    # boot oneshot above runs once, but some firmwares clear the limit when
+    # the charger is unplugged, so it must be re-asserted on the event.
+    # Matched by ATTR{type}=="Mains" (vendor-neutral — the kernel name
+    # AC/AC0/ADP1/ACAD varies); --no-block so the RUN+= returns at once
+    # (systemd kills long-running udev workers); restart (not try-restart)
+    # so it re-applies even if the boot run was inactive.
+    services.udev.extraRules = lib.mkIf chargeLimit ''
+      SUBSYSTEM=="power_supply", ATTR{type}=="Mains", RUN+="${config.systemd.package}/bin/systemctl --no-block restart nomarchy-battery-charge-limit.service"
+    '';
   };
 }