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.

modules/home/idle.nix

@@ -85,8 +85,9 @@ in
           # Suspend only on battery, and sooner than the old fixed 30 min
           # (it now only fires unplugged). Plugged in, the machine stays
           # up — long builds, media, presentations aren't killed mid-idle.
-          # Closing the lid still suspends on AC (logind's default): that's
-          # an explicit "I'm done", distinct from sitting idle.
+          # Lid close is logind's job (not hypridle): undocked lid still
+          # suspends (HandleLidSwitch); docked/clamshell lid is ignore
+          # (HandleLidSwitchDocked — modules/nixos/power.nix, #86).
           { timeout = 900; on-timeout = "${onAc} || systemctl suspend"; }
         ];
       };

modules/nixos/power.nix

@@ -31,6 +31,20 @@ in
     services.power-profiles-daemon.enable = lib.mkDefault ppd;
     services.tlp.enable = lib.mkDefault tlp;
 
+    # Clamshell / dock (BACKLOG #86): when the machine is "docked" in
+    # logind's sense (≥1 external display connected), closing the lid
+    # must NOT suspend — the external panel is the session. systemd's
+    # built-in default is already ignore; we set it explicitly so a
+    # downstream override or lock-bump drift is visible, and so
+    # checks.clamshell-logind can assert the contract.
+    # Lid alone (undocked) keeps the normal suspend path
+    # (HandleLidSwitch / ExternalPower left to upstream defaults).
+    # Display-profile "docked" layouts (eDP off) are orthogonal and
+    # already handled by nomarchy.displayProfiles.
+    services.logind.settings.Login = {
+      HandleLidSwitchDocked = lib.mkDefault "ignore";
+    };
+
     # thermald is Intel-only, so off unless asked (the installer enables
     # it on a GenuineIntel CPU). Sits happily next to either backend.
     services.thermald.enable = lib.mkDefault cfg.thermal.enable;