Make the settings screen redraw 50x faster

Arrow-key navigation took 435 ms per keypress, which reads as the whole console
reloading on every press - because it effectively was. The cost was forks: each
frame ran a command substitution per label for the value, the translation and
the rendered state, 54 subshells for eighteen rows.

The frame is now assembled in memory and written once. Translations, the split
specs and the terminfo clear string are resolved before the loop; values are
re-read only after something actually changes, not on cursor movement. Helpers
on that path assign to a variable instead of printing, since printing is what
forced the substitution.

Measured on the same machine: 435 ms -> 7.5 ms per keypress.

src/lib/common.sh

@@ -75,6 +75,27 @@ cau_msg() {
 	cau_msg_in "$(cau_ui_locale)" "$@"
 }
 
+# cau_msg_into <locale> <msgid>
+# Plain lookup with the result in CAU_MSG_RESULT and no printf formatting.
+# For callers that redraw many labels per keypress, where wrapping cau_msg in a
+# command substitution would cost a fork per label.
+CAU_MSG_RESULT=''
+
+cau_msg_into() {
+	local locale="$1" msgid="$2" cachekey
+	cachekey="${locale}"$'\x1f'"${msgid}"
+
+	if [[ -n ${CAU_MSG_CACHE[$cachekey]+set} ]]; then
+		CAU_MSG_RESULT="${CAU_MSG_CACHE[$cachekey]}"
+		return 0
+	fi
+
+	CAU_MSG_RESULT="$(LC_ALL="$locale" LANGUAGE="${locale%%.*}" gettext -- "$msgid" 2>/dev/null)"
+	[[ -n $CAU_MSG_RESULT ]] || CAU_MSG_RESULT="$msgid"
+	CAU_MSG_CACHE[$cachekey]="$CAU_MSG_RESULT"
+	return 0
+}
+
 # Translations are memoized. Every gettext lookup is a fork, and the settings
 # screen redraws forty-odd labels per keypress; without this the redraw takes
 # long enough that a keystroke arriving during it is lost when the terminal