fix(power): Dell charge limit needs Custom type, not Adaptive

charge_control_end_threshold was written (80) but charge_types stayed
[Adaptive], which ignores the cap while still reporting it — battery
kept charging past 80% on Latitude. Oneshot now selects Custom when a
limit is set, Adaptive/Standard when off; sets start hysteresis; doctor
warns if type ≠ Custom while limited.

V0: flake check --no-build green. V3: re-apply limit, confirm
charge_types shows [Custom] and status stops Charging above cap.

modules/nixos/options.nix

@@ -103,6 +103,8 @@
       batteryChargeLimit = lib.mkOption {
         type = lib.types.nullOr (lib.types.ints.between 50 100);
         default = config.nomarchy.settings.power.batteryChargeLimit or null;
+        # Dell Adaptive charge mode ignores the end threshold unless we
+        # also select Custom (power.nix oneshot); see Latitude 5310 QA.
         example = 80;
         description = ''
           Stop charging at this percentage to extend battery lifespan,
@@ -112,8 +114,10 @@
           null leaves charging at the firmware default (menu writes 100).
           Backend-independent: the menu applies live via sysfs (udev
           GROUP=users on the threshold node) and persists settings in
-          theme-state; a oneshot re-applies on boot and AC replug. Needs
-          nomarchy.system.power.laptop.
+          theme-state; a oneshot re-applies on boot and AC replug. On
+          Dell (and similar) the oneshot also selects charge type Custom
+          — Adaptive ignores the threshold while still reporting it.
+          Needs nomarchy.system.power.laptop.
         '';
       };
     };