fix(install): normalize empty keyboard variants

Treat the picker-only (none) row as an empty XKB variant at the shared input boundary and again in the template patcher. Guard the real downstream output and console keymap, and carry the sentinel through the offline installer VM.

Verified: V0 full flake evaluation plus shell/Python/Nix/diff checks; V1 installer-keyboard, installer-safety, and template-SoT builds; V2 full KVM offline LUKS+swap install and themed first boot. No V3 required.

agent/BACKLOG.md

@@ -30,22 +30,17 @@ 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.
 
-### 91. Installer: empty keyboard variant becomes literal `(none)` and blocks installation
-
-The generated system fails at `xkb-console-keymap.drv` with
-`WARNING: Can not find "(none)" in "us"` followed by ckbcomp's `bad include
-list`; every later system derivation is only a cascading dependency failure.
-Normalize the picker's “no variant” value to the empty string before writing
-Nix, and add an installer regression case for `us` + no variant. Pass = the
-generated console/XKB configuration builds and the install proceeds.
-
 ### 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. Keep the
-simple default path short. Pass = a user can deliberately choose US with no
-variant without knowing XKB terminology, and the generated values are shown
-before installation.
+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