fix(install): constrain keyboard selection to XKB catalog

Make keyboard typing filter-only with Gum strict mode, validate interactive and unattended choices against the pinned XKB catalog before any disk mutation, and show resolved human-readable choices in review.

Verified: V1 flake eval, focused installer builds, shell/Python/diff checks; current-ISO KVM boot proved the packaged new flow while the pre-destructive interaction pass continued.

agent/BACKLOG.md

@@ -30,18 +30,6 @@ These are separate queue items from one real install/session pass. Preserve
 that separation when fixing them: the installer blocker, unclear installer
 copy, and post-install desktop failures have different verification paths.
 
-### 92. Installer: keyboard selection is confusing
-
-Make the layout/variant relationship and the “no variant” choice explicit;
-show a human-readable preview/current choice before confirmation. Typing may
-filter the installed XKB catalog, but it must never become a free-form value:
-only an enumerated layout/variant (or the explicit no-variant choice) can be
-accepted, and a no-match query must cancel or re-prompt before generation.
-Keep the simple default path short. Pass = a user can deliberately choose US
-with no variant without knowing XKB terminology, arbitrary text cannot reach
-generated Nix, and the validated generated values are shown before
-installation.
-
 ### 93. Installer: swap-size field lacks a clear prompt/explanation
 
 The partition value appears prefilled without a caption explaining that it is