TTY pulse: constant layout, blue <-> dark grey (no more block-off)

The on/off pulse shifted the whole line by one character during the
dark frames (off-frame printed one space fewer than the block frame).
Per YuKu: keep the block ALWAYS present and alternate its colour
instead — blue (44) <-> dark grey (100, bright black). Layout is
constant, pulse is colour-based, nothing moves but the block itself.

Also: disable_selinux now mkdir -p's /etc/selinux before writing a
fresh config (harness caught a die on a missing parent dir).

lib/ui-block.sh

@@ -45,7 +45,7 @@ _ub_busy_loop() {
   depth=$(_ub_depth)
   case "$depth" in
     256) pal=(17 19 21 27 33 39 45 39 33 27 21 19) ;;  # deep breathe: dark navy to bright blue
-    16)  pal=(44 44 "" "") ;;                            # on/off pulse: console blues fuse
+    16)  pal=(44 44 100 100) ;;                          # blue <-> dark grey, constant layout
     *)   pal=("|" "/" "-" "\\") ;;                       # last-resort spinner
   esac
   while :; do
@@ -53,11 +53,7 @@ _ub_busy_loop() {
       if [[ "$depth" -eq 256 ]]; then
         _ub_paint "48;5;$i" "$_UB_MSG"       # 256-colour cube background
       elif [[ "$depth" -eq 16 ]]; then
-        if [[ -n "$i" ]]; then
-          _ub_paint "$i" "$_UB_MSG"          # plain ANSI background (blue)
-        else
-          printf '\r   %s' "$_UB_MSG" >&"$_UB_FD"   # block breathes out
-        fi
+        _ub_paint "$i" "$_UB_MSG"            # blue <-> dark grey pulse
       else
         printf '\r[%s] %s' "$i" "$_UB_MSG" >&"$_UB_FD"
       fi