Carry installer input-method selection into the target (#237)

* Carry the installer input-method choice into the target

* Support Archinstall 4.5 sanity checks

* Constrain VM keyboard choices to supported credential layouts

* Cover VM keyboard offsets and reject absent choices

test/unit/keyboard-selection-test.sh

@@ -0,0 +1,32 @@
+#!/bin/bash
+set -euo pipefail
+
+ROOT=$(cd -- "$(dirname -- "${BASH_SOURCE[0]}")/../.." && pwd)
+work=$(mktemp -d)
+trap 'rm -rf "$work"' EXIT
+mkdir -p "$work/install/provisioning"
+cat > "$work/install/provisioning/setup-form.sh" <<'FORM'
+OMARCHY_KEYBOARD_LAYOUTS=$'Chinese (Simplified, Pinyin)|us|pinyin
+English (US)|us
+Japanese|jp106|mozc'
+FORM
+export OMARCHY_PATH="$work"
+eval "$(sed -n '/^select_keyboard() {$/,/^}$/p' "$ROOT/bin/omarchy-iso-test")"
+press() { pressed+=("$1"); }
+sleep() { :; }
+
+pressed=()
+select_keyboard "Japanese"
+[[ ${pressed[*]} == "down" ]]
+pressed=()
+select_keyboard "Chinese (Simplified, Pinyin)"
+[[ ${pressed[*]} == "up" ]]
+pressed=()
+select_keyboard "English (US)"
+(( ${#pressed[@]} == 0 ))
+if (set -e; select_keyboard "Missing") 2>"$work/error"; then
+  echo "Unknown keyboard unexpectedly succeeded" >&2
+  exit 1
+fi
+grep -q 'Keyboard choice is absent' "$work/error"
+echo "ok - VM keyboard choices are relative to the default and reject absent rows"