feat(waybar): show the layout indicator only when the layout is ambiguous (#109)

The indicator answers "which layout am I typing in?", so it is worth bar
space exactly when that question has more than one answer. It was gated on a
per-device override merely *existing*, which is the trap this box walked
into: the junk rows a keyboard's extra HID collections leave behind are all
remembered at the session layout, so they added no second answer and still
raised an indicator that could only ever show one value.

Gate on layouts rather than devices, and read the runtime-remembered
settings.keyboard.devices alongside the declared option — the new-keyboard
watcher writes those without a rebuild and they only graduate into
keyboard.devices later, so a bar reading the option alone was wrong for
exactly as long as the memory was fresh, which is when it matters most.

keyboard.layouts is now excluded on purpose: hyprland.nix keeps it as the
pool the picker offers first and deliberately never merges it into kb_layout,
so listing candidates cannot make the current layout ambiguous. A device
actually using one of them still does.

All four whole-swaps list the module unconditionally, so a single-layout
setup saw on a swapped bar what the generated bar hides — the parity rule was
already broken. Filter a swap through the same answer instead of asking four
hand-written files to re-derive it.

Logic is pure in modules/home/waybar-language.nix, matching monitor-rules /
dock-audio-rules; checks.waybar-language unit-tests both halves against
boreal's real swap, including a fixture-drift guard so the parity assertion
cannot pass vacuously.

Verified V2: new check passes, docking-ux and theme-wholeswap still pass,
`nix flake check` is clean, and the live config's generated bar now resolves
modules-right without hyprland/language on its single-layout default.

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

modules/home/waybar-language.nix

@@ -0,0 +1,51 @@
+# When the keyboard-layout indicator earns a place in the bar, and how a
+# whole-swap bar is held to the same answer (BACKLOG #109).
+#
+# Pure so checks.waybar-language can unit-test the contract without building
+# a bar. Consumed by modules/home/waybar.nix.
+{ lib }:
+
+rec {
+  # The session's layouts are the comma-separated kb_layout.
+  # nomarchy.keyboard.layouts is deliberately NOT among them — hyprland.nix
+  # keeps it as the pool the new-keyboard picker offers first and never
+  # merges it into kb_layout — so listing candidates cannot by itself make
+  # the current layout ambiguous.
+  sessionLayouts = layout: lib.filter (l: l != "") (lib.splitString "," layout);
+
+  # The indicator answers "which layout am I typing in?", so it is worth bar
+  # space exactly when that has more than one answer.
+  #
+  # devices    = declared nomarchy.keyboard.devices (submodules with .layout)
+  # remembered = settings.keyboard.devices from the in-flake state: what the
+  #              new-keyboard watcher writes the moment a layout is chosen.
+  #              These only graduate into `devices` at the next rebuild, so a
+  #              bar reading `devices` alone stays wrong for exactly as long
+  #              as the memory is fresh — which is when it matters most.
+  #
+  # Counting devices rather than layouts is the trap: remembering "us" for a
+  # device on a "us" session adds no second answer, and the rows a keyboard's
+  # extra HID collections leave behind are all of that kind.
+  show = { layout, devices ? { }, remembered ? { } }:
+    let
+      session = sessionLayouts layout;
+      perDevice = lib.mapAttrsToList (_: d: d.layout) devices
+        ++ lib.attrValues remembered;
+    in
+    lib.length session > 1 || lib.any (l: ! lib.elem l session) perDevice;
+
+  # A whole-swap authors its bar in full and every one of them lists the
+  # language module unconditionally, so without this a single-layout setup
+  # showed on a swapped bar what the generated bar hides. Filter the swap
+  # through the one answer rather than asking four hand-written files to
+  # re-derive it (CONVENTIONS: the parity rule).
+  gate = showLanguage: bar:
+    if showLanguage then bar
+    else
+      lib.mapAttrs
+        (n: v:
+          if lib.hasPrefix "modules-" n && builtins.isList v
+          then lib.filter (m: m != "hyprland/language") v
+          else v)
+        bar;
+}

modules/home/waybar.nix

@@ -12,12 +12,16 @@
 let
   t = config.nomarchy.theme;
 
-  # Show the active-keyboard-layout indicator only when more than one layout
-  # is in play — multiple session layouts (comma-separated) or per-device
-  # overrides (nomarchy.keyboard.devices) — so single-layout bars stay clean.
-  showLanguage = lib.hasInfix "," config.nomarchy.keyboard.layout
-    || config.nomarchy.keyboard.layouts != [ ]
-    || config.nomarchy.keyboard.devices != { };
+  # Whether the keyboard-layout indicator is worth bar space, and the filter
+  # that holds a whole-swap to the same answer. Contract + rationale (and
+  # checks.waybar-language) live in ./waybar-language.nix.
+  langLib = import ./waybar-language.nix { inherit lib; };
+  showLanguage = langLib.show {
+    layout = config.nomarchy.keyboard.layout;
+    devices = config.nomarchy.keyboard.devices;
+    remembered = config.nomarchy.settings.keyboard.devices or { };
+  };
+  gateLanguage = langLib.gate showLanguage;
 
   # Power-profile indicator (power-profiles-daemon). Both scripts self-
   # gate: they exit silently unless this is a laptop (a battery is
@@ -534,7 +538,7 @@ in
     # docs/OVERRIDES.md.
     settings.mainBar = lib.mkDefault (
       if hasConfigOverride
-      then builtins.fromJSON (builtins.readFile configOverride)
+      then gateLanguage (builtins.fromJSON (builtins.readFile configOverride))
       else generatedSettings
     );