feat(power): clamshell — ignore lid suspend when docked (#86)

Explicit HandleLidSwitchDocked=ignore under services.logind so closing
the lid with an external display connected does not suspend. Undocked
lid path left to upstream defaults. checks.clamshell-logind asserts the
baked logind.conf and active logind; real multi-monitor lid V3 queued.

Verified: V2 — flake check --no-build; checks.clamshell-logind green.
V3 pending: HARDWARE-QUEUE #86 lid+external on a laptop.

flake.nix

@@ -510,6 +510,33 @@
             '';
           };
 
+          # Clamshell / dock lid policy (#86): logind must ignore lid close
+          # when docked (external display present). Real multi-monitor lid
+          # events stay V3; this asserts the baked logind.conf + that
+          # logind is alive to consume it. Imports options + power only.
+          clamshell-logind = pkgs.testers.runNixOSTest {
+            name = "nomarchy-clamshell-logind";
+            nodes.machine = { ... }: {
+              imports = [ ./modules/nixos/options.nix ./modules/nixos/power.nix ];
+              nomarchy.system.power = {
+                enable = true;
+                laptop = true;
+              };
+            };
+            testScript = ''
+              machine.wait_for_unit("multi-user.target")
+              machine.wait_for_unit("systemd-logind.service")
+              conf = machine.succeed("cat /etc/systemd/logind.conf")
+              assert "HandleLidSwitchDocked=ignore" in conf.replace(" ", ""), (
+                  f"clamshell: logind.conf missing HandleLidSwitchDocked=ignore:\n{conf}"
+              )
+              # logind must be running (policy is dead if the unit is down).
+              machine.succeed("systemctl is-active systemd-logind.service")
+              # busctl property: Docked lid policy is not always exported;
+              # re-read conf after logind start is enough for the contract.
+            '';
+          };
+
           # Runtime VM check for the battery-charge-limit re-apply: the udev
           # rule must restart the oneshot on an AC state change (firmware can
           # clear the threshold on unplug). The `test_power` module fakes a