feat(keyboard): multi-layout cycle bind + summer-bar language parity

SUPER+SHIFT+K → hyprctl switchxkblayout current next, rendered only
when the session layout has a comma — the same gate as the Waybar
language indicator. Bind data is a separate multiLayoutBinds list in
keybinds.nix (still one source); hyprland.nix and the rofi cheatsheet
gate on the identical condition, so bind and cheatsheet cannot drift.
`current` targets the focused keyboard, so per-device-overridden
boards (single layout) are a no-op.

Parity: hyprland/language (with the keyboard glyph) added to both
summer waybar.jsonc whole-swaps + #language in their CSS. Static JSON
cannot eval-gate, so on summer themes the module shows even with one
layout — parity-over-minimalism, deliberate. Found en route:
summer-night has an idle_inhibitor the generated bar lacks (reverse
gap) — folded into backlog item 13.

Also — agent/BACKLOG.md gains items 21-24 from Bernardo's Latitude
5410 hardware QA (waybar crash on theme switch + no respawn; nameless
network-menu rows; a no-update sys-rebuild; Back-everywhere audit),
queued ahead of everything; HARDWARE-QUEUE's snapshot re-test now
carries the "update to >= a47aa3a + relogin" precondition, since
findings 5/6 match the pre-polkit-agent behavior exactly.

Verified: V0 (flake check, both summer JSONs parse) + V1 both
directions (default single-layout build renders no bind; us,de via
extendModules renders bind AND cheatsheet row). V3 cycle check queued.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

docs/ROADMAP.md

@@ -721,11 +721,23 @@ how to override it. Items marked ✓ are shipped.
     which the watcher sees it as declared and steps back. Eval-verified
     (graduation + precedence) and the writer round-trips; **still needs the
     on-hardware hotplug re-verify** (the picker path isn't testable in CI).
-  - 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).
+  - ✓ **Multi-layout cycle bind + summer parity (2026-07-04):**
+    `SUPER+SHIFT+K` → `hyprctl switchxkblayout current next`, rendered
+    only when the session layout has a comma (same gate as the Waybar
+    language indicator) — data lives in `keybinds.nix` as a separate
+    `multiLayoutBinds` list, so hyprland.nix and the cheatsheet consume
+    one source and gate identically (verified both ways: absent on a
+    single layout; bind + cheatsheet row render under `us,de` via
+    extendModules). `current` targets the focused keyboard, so a
+    per-device-overridden board (one layout) is a no-op, never a leak.
+    `hyprland/language` (`󰌌 {short}`) added to both summer
+    `waybar.jsonc` whole-swaps + `#language` in their CSS — static JSON
+    can't eval-gate, so on summer themes the module shows even with one
+    layout (a deliberate parity-over-minimalism call; it's small).
+    Noticed en route: summer-night carries an `idle_inhibitor` the
+    generated bar lacks — folded into the idle-inhibit backlog item.
+  - Remaining: 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