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/config.sh

@@ -20,11 +20,17 @@ _cau_config_slurp() {
 	return 0
 }
 
-# cau_config_get <Key> [default]
-cau_config_get() {
+# _cau_config_lookup <Key> [default]
+# Result in CAU_CONFIG_VALUE. Assigning rather than printing matters on the
+# settings screen, which reads every key on every frame: a command substitution
+# there is a fork, and forks were the entire cost of a redraw.
+CAU_CONFIG_VALUE=''
+
+_cau_config_lookup() {
 	local key="$1" default="${2:-}" val='' line
 
-	[[ -r $CAU_CONFIG ]] || { printf '%s\n' "$default"; return; }
+	CAU_CONFIG_VALUE="$default"
+	[[ -r $CAU_CONFIG ]] || return 0
 	_cau_config_slurp
 
 	# last assignment wins, matching the previous sed|tail behaviour
@@ -41,11 +47,14 @@ cau_config_get() {
 	val="${val%\"}"
 	val="${val#\"}"
 
-	if [[ -n $val ]]; then
-		printf '%s\n' "$val"
-	else
-		printf '%s\n' "$default"
-	fi
+	[[ -n $val ]] && CAU_CONFIG_VALUE="$val"
+	return 0
+}
+
+# cau_config_get <Key> [default]
+cau_config_get() {
+	_cau_config_lookup "$@"
+	printf '%s\n' "$CAU_CONFIG_VALUE"
 }
 
 # cau_config_bool <Key> <default: yes|no>