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>

themes/summer-day/waybar.css

@@ -49,6 +49,7 @@ window#waybar {
 #pulseaudio,
 #custom-powerprofile,
 #custom-nightlight,
+#language,
 #custom-updates,
 #battery,
 #tray,