feat(sway): 4K scale 2, natural mouse scroll, idle inhibit for emulators

Three unrelated desktop-side fixes, all in home/sway.nix.

Sway cannot key an output rule on resolution, so scale 2 on 4K displays
is applied by a user service that runs once at session start and again on
every output event, which is what makes it survive a display being
switched off and on. The scale guard in the jq filter stops the
subscription feeding itself. kanshi still owns mode and position; no
profile here sets a scale, so the two do not fight.

natural_scroll on type:pointer gives mice the same scroll direction as
the touchpads already had.

PCSX2 and Dolphin default to borderless-windowed rather than compositor
fullscreen, so the existing inhibit_idle fullscreen rules never matched
and swaylock engaged mid-game. Inhibit on focus for the emulators
instead. The Steam session is unaffected either way: swayidle is wanted
by graphical-session.target and conditional on WAYLAND_DISPLAY, and the
gamescope session provides neither.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

home/sway.nix

@@ -30,6 +30,28 @@ let
       - | ${pkgs.swappy}/bin/swappy -f -
   '';
 
+  # Sway's config cannot express an output rule keyed on resolution, so scale is
+  # applied from outside it: once at session start, then on every output event,
+  # which is what makes it survive a display being unplugged or power-cycled.
+  # The `.scale != 2` guard stops the subscription feeding itself, since setting
+  # a scale is itself an output event.
+  scaleHidpi = pkgs.writeShellScript "sway-scale-hidpi" ''
+    apply() {
+      ${pkgs.sway}/bin/swaymsg -r -t get_outputs \
+        | ${pkgs.jq}/bin/jq -r '.[]
+            | select(.active and .current_mode.width >= 3840 and .scale != 2)
+            | .name' \
+        | while read -r name; do
+            ${pkgs.sway}/bin/swaymsg output "$name" scale 2
+          done
+    }
+
+    apply
+    ${pkgs.sway}/bin/swaymsg -t subscribe -m '["output"]' | while read -r _; do
+      apply
+    done
+  '';
+
   # Binding-mode names. The string is both the `modes` attr key and what the
   # bar's mode indicator shows, so the keys are spelled out in the label.
   layoutMode = "layout:  [s]tacking  [w]tabbed  [e]split";
@@ -60,6 +82,8 @@ in
         tap = "enabled";
         natural_scroll = "enabled";
       };
+      # Mice (the MX Master 3 among them) scroll the same way as the touchpads.
+      input."type:pointer".natural_scroll = "enabled";
 
       # Solid Catppuccin Mocha base as the wallpaper (no image dependency).
       output."*".bg = "#${ctp.base} solid_color";
@@ -111,6 +135,18 @@ in
           criteria.class = ".*";
           command = "inhibit_idle fullscreen";
         }
+        # Emulators default to borderless-windowed rather than compositor
+        # fullscreen, so the rules above never fire and swaylock cuts in
+        # mid-game. Inhibit on focus instead, which does not depend on how the
+        # application asked to fill the screen.
+        {
+          criteria.app_id = "(?i)^(pcsx2|dolphin-emu|retroarch|clone.?hero)";
+          command = "inhibit_idle focus";
+        }
+        {
+          criteria.class = "(?i)^(pcsx2|dolphin-emu|retroarch|clone.?hero)";
+          command = "inhibit_idle focus";
+        }
       ];
 
       # Binding modes (submenus). Entered from keybindings below; each action
@@ -307,6 +343,24 @@ in
     Install.WantedBy = [ "sway-session.target" ];
   };
 
+  # Every 4K output runs at scale 2 (1920x1080 logical), on every host and
+  # whatever the display is called. kanshi below owns mode and position; this
+  # owns scale for the one case that is a fleet-wide rule rather than a
+  # per-display fact, and the two do not fight because no kanshi profile here
+  # sets a scale.
+  systemd.user.services.sway-scale-hidpi = {
+    Unit = {
+      Description = "Force scale 2 on 4K Sway outputs";
+      PartOf = [ "graphical-session.target" ];
+      After = [ "graphical-session.target" ];
+    };
+    Service = {
+      ExecStart = "${scaleHidpi}";
+      Restart = "on-failure";
+    };
+    Install.WantedBy = [ "sway-session.target" ];
+  };
+
   # Output/display management. Reacts to hotplug and applies per-display
   # mode/scale/position. Profiles are hardware-specific: the safe default below
   # just enables the internal laptop panel; add docked/desktop profiles with the