feat(power): laptop power management (power-profiles-daemon + TLP)

Adds a `nomarchy.system.power.*` surface (modules/nixos/power.nix),
replacing the bare `services.upower` (Waybar reporting only) baseline:

- power-profiles-daemon ships by default; switched through polkit (no
  root prompt). TLP is the opt-in `backend = "tlp"` for deeper battery
  tuning — the two are mutually exclusive (asserted).
- thermald behind `power.thermal.enable` (Intel-only).
- `power.batteryChargeLimit` (e.g. 80): a backend-independent sysfs
  oneshot writes charge_control_end_threshold; gated on `power.laptop`.

Switcher + indicator live home-side and self-gate on a battery being
present + powerprofilesctl running, so they hide on desktops and under
TLP — no system->home wiring (like the Waybar battery widget):

- rofi.nix: `nomarchy-menu power-profile` module + a picker entry shown
  only on laptops with PPD.
- waybar.nix: first `custom/*` module — click cycles the profile.

Installer writes `power.laptop = true` (battery probe) and
`power.thermal.enable` (GenuineIntel) into the generated system.nix, and
scaffolds the charge limit commented-out.

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

modules/nixos/options.nix

@@ -51,5 +51,58 @@
       `nixos-rebuild-snap` pre-rebuild-snapshot helper. No-op unless the
       root filesystem is BTRFS with a /.snapshots subvolume (the installer
       creates one)'';
+
+    power = {
+      enable = lib.mkEnableOption ''
+        active power management. By default ships power-profiles-daemon —
+        the upstream-aligned power-saver/balanced/performance model,
+        switchable from the menu (nomarchy-menu power-profile) and shown
+        in Waybar on laptops — plus thermald and a battery charge limit
+        where applicable. Harmless on desktops (the profile daemon just
+        offers balanced/performance)'' // { default = true; };
+
+      backend = lib.mkOption {
+        type = lib.types.enum [ "ppd" "tlp" ];
+        default = "ppd";
+        description = ''
+          Which daemon governs CPU/platform power. "ppd"
+          (power-profiles-daemon) is the default: a clean three-profile
+          model with the menu switcher and Waybar indicator. "tlp" trades
+          that for TLP's deeper, more aggressive battery tuning, at the
+          cost of the profile switcher (TLP has no profile concept). The
+          two manage the same knobs and are mutually exclusive.
+        '';
+      };
+
+      laptop = lib.mkOption {
+        type = lib.types.bool;
+        default = false;
+        description = ''
+          Marks this machine as a laptop, gating battery-only features
+          (the charge limit below). The installer sets it to true when it
+          detects a battery at install time.
+        '';
+      };
+
+      thermal.enable = lib.mkEnableOption ''
+        thermald, Intel's thermal-management daemon, to avoid aggressive
+        throttling under sustained load. Intel-only — the installer
+        enables it when it detects a GenuineIntel CPU. Harmless alongside
+        either backend'';
+
+      batteryChargeLimit = lib.mkOption {
+        type = lib.types.nullOr (lib.types.ints.between 50 100);
+        default = null;
+        example = 80;
+        description = ''
+          Stop charging at this percentage to extend battery lifespan,
+          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.
+        '';
+      };
+    };
   };
 }