TTY: on/off block pulse on the Linux console (shade fusion defeated)

YuKu tests on the bare Linux VT (not a terminal emulator): my 16-colour
fallback alternated blue (44) / bright blue (104), which render nearly
identically on VGA consoles -> fused into 'solid blue'. If TERM
misreports 256, 48;5;N codes are ignored outright.

Fixes:
- TERM linux*/console* forces the 16-colour path regardless of tput.
- 16-colour mode now PULSES THE BLOCK ON/OFF (2 frames blue block,
  2 frames no block, ~2.5Hz) instead of shade alternation — visible
  on any console hardware, no colour fidelity required.
- 256-colour terminals keep the deep 17->45 breathe ramp.

Verified: TERM=linux pty shows block/no-block alternation with zero
48;5 codes; TERM=xterm-256color keeps the 8-shade ramp; unattended +
piped regressions green.

lib/ui-block.sh

@@ -31,6 +31,8 @@ _ub_depth() {
   if   [[ "$n" -ge 256 ]]; then echo 256
   elif [[ "$n" -ge 8   ]]; then echo 16
   else echo 0; fi
+  # bare Linux VT: cap regardless of what tput reports
+  case "${TERM:-}" in linux*|console*) echo 16 ;; esac
 }
 
 # paint one background-coloured block (a SPACE) + message
@@ -43,7 +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=(44 104 44 104) ;;                          # blue / bright blue
+    16)  pal=(44 44 "" "") ;;                            # on/off pulse: console blues fuse
     *)   pal=("|" "/" "-" "\\") ;;                       # last-resort spinner
   esac
   while :; do
@@ -51,7 +53,11 @@ _ub_busy_loop() {
       if [[ "$depth" -eq 256 ]]; then
         _ub_paint "48;5;$i" "$_UB_MSG"       # 256-colour cube background
       elif [[ "$depth" -eq 16 ]]; then
-        _ub_paint "$i" "$_UB_MSG"            # plain ANSI background (44/104)
+        if [[ -n "$i" ]]; then
+          _ub_paint "$i" "$_UB_MSG"          # plain ANSI background (blue)
+        else
+          printf '\r   %s' "$_UB_MSG" >&"$_UB_FD"   # block breathes out
+        fi
       else
         printf '\r[%s] %s' "$i" "$_UB_MSG" >&"$_UB_FD"
       fi