feat(battery): low-battery notifications at the bar's 25/10% thresholds (item 13 slice)

The bar colored the battery at 25/10% but nothing notified. New
shellcheck-gated watcher (pkgs/nomarchy-battery-notify, overlay) polls
the same sysfs the bar reads — type=Battery, scope!=Device — and fires
one toast per downward crossing (normal at 25%, critical at 10%; swaync
keeps critical up until dismissed), re-armed by charging. Self-gates on
battery presence (the powerprofile pattern), so desktops get a silent
no-op. HM user unit behind nomarchy.batteryNotify.enable (default on)
with libnotify on the unit PATH; notify-send resolves from PATH so the
VM check can shim it. poweralertd rejected: its UPower thresholds
(20/5) wouldn't match the bar, and it toasts every plug/unplug.

Verified: V0 (flake check, bash -n); V1 (template-home builds, unit in
the generation); V2 GREEN (new checks.battery-notify, test_power fake
battery: self-gate, silence at 80%, one toast per crossing, critical
urgency, charging re-arms). Remains: V3 — real swaync toast on a
draining laptop (HARDWARE-QUEUE).

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

flake.nix

@@ -84,6 +84,7 @@
           themesDir = ./themes;
         };
         nomarchy-doctor = final.callPackage ./pkgs/nomarchy-doctor { };
+        nomarchy-battery-notify = final.callPackage ./pkgs/nomarchy-battery-notify { };
       };
 
       nixosModules.nomarchy = {
@@ -331,6 +332,68 @@
             '';
           };
 
+          # The low-battery watcher's crossing logic: one toast per downward
+          # crossing (normal at 25%, critical at 10%), re-armed by charging.
+          # test_power fakes a battery whose capacity/status we script;
+          # notify-send is shimmed to a log via PATH (the package resolves
+          # it from PATH for exactly this reason). What stays on-hardware:
+          # the real toast rendering through swaync in a session.
+          battery-notify = pkgs.testers.runNixOSTest {
+            name = "nomarchy-battery-notify";
+            nodes.machine = { ... }: {
+              boot.kernelModules = [ "test_power" ]; # fake battery + Mains
+              environment.systemPackages = [ pkgs.nomarchy-battery-notify ];
+            };
+            testScript = ''
+              machine.wait_for_unit("multi-user.target")
+
+              # notify-send shim: log the calls instead of needing a session bus.
+              machine.succeed(
+                  "mkdir -p /shim && printf '#!/bin/sh\necho \"$*\" >> /tmp/notifications\n'"
+                  " > /shim/notify-send && chmod +x /shim/notify-send"
+              )
+
+              # Self-gate: with the fake battery gone, the watcher exits 0 at once.
+              machine.succeed("modprobe -r test_power")
+              machine.succeed("timeout 5 nomarchy-battery-notify 1")
+              machine.succeed("modprobe test_power")
+
+              # Healthy level while discharging: silence.
+              machine.succeed("echo discharging > /sys/module/test_power/parameters/battery_status")
+              machine.succeed("echo 80 > /sys/module/test_power/parameters/battery_capacity")
+              machine.succeed(
+                  "systemd-run --unit=batwatch --setenv=PATH=/shim:/run/current-system/sw/bin"
+                  " /run/current-system/sw/bin/nomarchy-battery-notify 1"
+              )
+              machine.sleep(3)
+              machine.fail("test -s /tmp/notifications")
+
+              # Crossing 25% fires the low toast exactly once, not per poll.
+              machine.succeed("echo 20 > /sys/module/test_power/parameters/battery_capacity")
+              machine.wait_until_succeeds("grep -q 'Battery low' /tmp/notifications")
+              machine.succeed("echo 15 > /sys/module/test_power/parameters/battery_capacity")
+              machine.sleep(3)
+              assert machine.succeed("grep -c 'Battery low' /tmp/notifications").strip() == "1"
+
+              # Crossing 10% escalates to a critical-urgency toast, once.
+              machine.succeed("echo 5 > /sys/module/test_power/parameters/battery_capacity")
+              machine.wait_until_succeeds("grep -q 'Battery critical' /tmp/notifications")
+              assert "critical" in machine.succeed("grep 'Battery critical' /tmp/notifications")
+              machine.sleep(3)
+              assert machine.succeed("grep -c 'Battery critical' /tmp/notifications").strip() == "1"
+
+              # Back on the charger → re-armed: the next drain notifies again.
+              machine.succeed("echo charging > /sys/module/test_power/parameters/battery_status")
+              machine.succeed("echo 80 > /sys/module/test_power/parameters/battery_capacity")
+              machine.sleep(3)
+              machine.succeed("echo discharging > /sys/module/test_power/parameters/battery_status")
+              machine.succeed("echo 20 > /sys/module/test_power/parameters/battery_capacity")
+              machine.wait_until_succeeds(
+                  "test \"$(grep -c 'Battery low' /tmp/notifications)\" = 2"
+              )
+            '';
+          };
+
           # Memory-pressure protection (oom.nix): earlyoom must kill a
           # runaway allocator BEFORE the kernel thrash point — and must not
           # take anything else with it. A bystander unit survives the kill,