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/MEMORY.md

@@ -139,6 +139,8 @@ iteration would otherwise rediscover.
   without a hierarchy pass.
 
 ## Gotchas (cost a debugging session once)
+- Gum `filter` returns unmatched typed text by default; catalog-only pickers
+  require `--strict` plus an independent exact-membership validation boundary.
 - Waybar `layer: top` renders above **even real-fullscreen windows** — the
   bar draws over a fullscreen video. `layer: bottom` lets the fullscreen
   surface cover it while the exclusive zone still reserves the bar's space