feat(installer): staged banner reveal at 40ms/line

Read banner.txt line by line with a 40ms sleep between each, then
clear and redraw the bordered gum-style version. 5-line banner ×
40ms = 200ms total reveal — slow enough to land an aesthetic on the
first frame, fast enough that the operator never feels it as lag.

Pure cosmetic; no functional change to the install flow.

overlay/usr/local/bin/veilor-installer

@@ -69,6 +69,21 @@ banner() {
     local vline="veilor-os ${ver} · ${d} · live"
 
     if [[ -r $BANNER_FILE ]]; then
+        # v0.6: staged line-by-line reveal of the banner before the
+        # gum-style border draws around it. 40ms/line gives a subtle
+        # "typewriter" feel — 5 lines × 40ms = 200ms total, fast enough
+        # not to feel laggy but slow enough to land an aesthetic on the
+        # very first frame the user sees. Once the reveal finishes we
+        # clear and re-draw with the bordered gum-style version so the
+        # operator never sees both stacked on top of each other.
+        local line
+        while IFS= read -r line; do
+            printf '  %s\n' "$line"
+            sleep 0.04
+        done < "$BANNER_FILE"
+        sleep 0.08
+        clear
+
         if [[ $TUI == gum ]]; then
             # gum style: rounded border, banner + blank line + version line.
             gum style --border rounded --margin "0 2" --padding "1 3" \