feat(keyboard): per-device layout for external keyboards

Add nomarchy.keyboard.devices ({ "<hyprctl-device-name>" = { layout; variant; }; })
generating Hyprland `device` blocks that override the session
nomarchy.keyboard.layout for a named keyboard -- e.g. an external board
that's physically a different layout than the laptop's built-in one.
Hyprland applies it whenever that device connects, so the external keyboard
"remembers" its layout the declarative way. A Waybar hyprland/language
indicator shows the active layout, placed only when more than one layout is
in play (a comma in the session layout, or any per-device override) so
single-layout bars stay clean.

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

docs/ROADMAP.md

@@ -283,11 +283,21 @@ how to override it. Items marked ✓ are shipped.
 - **Night light / blue-light filter:** schedulable colour-temperature shift
   via `hyprsunset` (Hyprland-native; wlsunset/gammastep are alternatives) —
   sunset/sunrise or a fixed schedule, with a menu + Waybar toggle.
-- **Runtime keyboard-layout switching:** the session layout is a single
-  `nomarchy.keyboard.layout` today. Support a list of layouts with a toggle
-  bind (`hyprctl switchxkblayout` / xkb `grp:` options) and a Waybar
-  indicator for the active layout. Natural follow-on to the LUKS-keymap work
-  — keep the system (console/initrd) and session layouts in sync.
+- **Keyboard layouts (per-device + switching):**
+  - ✓ **Per-device declarative layout:** `nomarchy.keyboard.devices`
+    (`{ "<hyprctl-device-name>" = { layout; variant; }; }`) generates Hyprland
+    `device` blocks that override the session `nomarchy.keyboard.layout` for a
+    named keyboard — e.g. an external board that's physically a different
+    layout than the laptop's. Hyprland applies it whenever that device
+    connects (the "external keyboard remembers its layout" case, done the
+    declarative way). A Waybar `hyprland/language` indicator shows the active
+    layout, gated on more than one layout being in play (comma in the session
+    layout, or any per-device override) so single-layout bars stay clean.
+  - Remaining: a multi-layout cycle bind (`hyprctl switchxkblayout` / xkb
+    `grp:` options) for switching layouts on one keyboard; add the
+    `hyprland/language` module to the summer whole-swap themes for parity
+    (the gating doesn't translate to their static JSON). Keep the system
+    (console/initrd) and session layouts in sync (the LUKS-keymap work).
 - ✓ **Do-Not-Disturb:** swaync DND toggle wired into the menu
   (`nomarchy-menu dnd`, in the picker, SUPER+CTRL+D) and a Waybar bell
   indicator (`custom/notification` via `swaync-client -swb`: shows the