feat(hyprland): add runtime monitor left/right swap bind

`$mod CTRL o` reflows the two connected monitors adjacent in reversed
order (gap-free regardless of differing widths) and re-inits the
layer-shell clients, which Hyprland doesn't reliably re-notify on a
runtime output move: waybar rebuilds on SIGUSR2, wallpaper is re-pushed.
`monitor` default becomes `mkDefault` so a host can pin its own layout.

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

modules/home.nix

@@ -79,6 +79,47 @@ let
       2) ${pkgs.systemd}/bin/systemctl poweroff ;;
     esac
   '';
+
+  # Swap the physical left/right placement of a two-monitor setup at runtime.
+  # Reflows the pair adjacent from x=0 (left one at origin, right one butted
+  # against its logical width) after reversing their order — so it stays
+  # gap-free regardless of differing widths, unlike a naive x-origin exchange
+  # (equal widths only). Machine-agnostic: reads whichever two connectors are
+  # live from `hyprctl -j`, no connector names. Ephemeral (hyprctl keyword) —
+  # the per-host default layout is restored on the next reload/relogin. No-op
+  # unless exactly two monitors are connected.
+  #
+  # Hyprland doesn't reliably re-notify layer-shell clients when an output is
+  # reconfigured at runtime (a known limitation — same class as the mirror/unplug
+  # cases where waybar needs a restart). Observed here: after the move, waybar and
+  # the wallpaper keep drawing at the pre-swap offsets, so the moved monitor's
+  # bar/bg land off-screen. Re-init both against the new geometry: waybar rebuilds
+  # its bars on SIGUSR2, and awww only paints outputs present when `awww img` last
+  # ran, so re-push the wallpaper symlink. Both are idempotent — harmless when a
+  # surface happened not to lag (which surface stales seems to race).
+  swapMonitors = pkgs.writeShellScript "hypr-swap-monitors" ''
+    set -eu
+    hc=${config.wayland.windowManager.hyprland.finalPackage}/bin/hyprctl
+    jq=${pkgs.jq}/bin/jq
+    data=$("$hc" monitors -j)
+    n=$(printf '%s' "$data" | "$jq" length)
+    if [ "$n" -ne 2 ]; then
+      ${pkgs.libnotify}/bin/notify-send "Monitor swap" "need exactly 2 monitors ($n connected)"
+      exit 0
+    fi
+    # sort by current x → [left, right]; the new left is the old right.
+    cmd=$(printf '%s' "$data" | "$jq" -r '
+      sort_by(.x)
+      | .[1] as $l | .[0] as $r
+      | (($l.width / $l.scale) | floor) as $off
+      | "keyword monitor \($l.name),preferred,0x0,\($l.scale)"
+      + " ; keyword monitor \($r.name),preferred,\($off)x0,\($r.scale)"')
+    "$hc" --batch "$cmd"
+    # re-anchor the layer-shell clients onto the new geometry (see comment above)
+    ${pkgs.procps}/bin/pkill -USR2 -x .waybar-wrapped || true
+    bg="$HOME/.local/share/bg"
+    [ -e "$bg" ] && ${pkgs.awww}/bin/awww img "$bg" || true
+  '';
 in
 {
   imports = [ ./gpg.nix ./media.nix ./texlive.nix ./librewolf.nix ./mc.nix ./mime.nix ./hyprland-workspaces.nix ];
@@ -479,7 +520,10 @@ in
     configType = "hyprlang";
 
     settings = {
-      monitor = ",preferred,auto,1";
+      # Fallback for hosts that don't pin a layout; a two-monitor host overrides
+      # this with explicit positions (see the private inventory). mkDefault so
+      # that override is a plain value, not a mkForce fight.
+      monitor = lib.mkDefault ",preferred,auto,1";
 
       # The qt module sets QT_QPA_PLATFORMTHEME=gtk3 (→ Qt apps read the GTK
       # Adwaita-dark theme), but only into hm-session-vars.sh + the systemd user
@@ -582,6 +626,8 @@ in
         # With two monitors the relative +1 just toggles between them.
         "$mod,		o,	focusmonitor,			+1"
         "$mod SHIFT,	o,	movecurrentworkspacetomonitor,	+1"
+        # Swap the two monitors' left/right placement (see swapMonitors above).
+        "$mod CTRL,	o,	exec,	${swapMonitors}"
       ] ++ (
         # Dynamic definitions for viewing workspaces 1 to 9 and moving windows between them.
         # References physical key codes for robustness with the SHIFT key, e.g. `code:10` instead of `1`.