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>

agent/BACKLOG.md

@@ -149,13 +149,6 @@ Hyprland binding appears from the canonical source. Do not repeat the obsolete
 claim that float/move bindings are absent. Pass = presentation is scannable and
 an automated comparison detects omissions or stale displayed bindings.
 
-### 109. Conditional keyboard Waybar indicator parity
-
-Hide the keyboard indicator for an unambiguous one-layout setup; show it when
-multiple layouts, a per-device override, or a remembered keyboard makes it
-useful. Pass = hotplug/remembered-device transitions update correctly and every
-whole-bar swap matches the generated default bar.
-
 ### 110. `[big]` Retire Control Center safely
 
 Split this into two phases before implementation: first build a migration/drop