Screenshots: 10-inch tablet set; guard against blank captures

The 7-inch roots capture was pure app background (Compose had not composed its
first frame yet, though the system bars had drawn) so it measured as uniform.
made-store-screenshots.sh now measures each capture and refuses to normalise a
blank one.

tools/make-store-screenshots.sh

@@ -34,6 +34,15 @@ esac
 status_px="${STATUS_BAR_PX:-$default_status}"
 nav_px="${NAV_BAR_PX:-$default_nav}"
 
+# A headless AVD whose display has slept, or an app whose first frame has not composed yet,
+# produces a screencap of pure background. The top and bottom system bars are still drawn,
+# so the file is not *obviously* empty — but the app area has no ink at all. That is
+# invisible in a thumbnail and unacceptable on a listing, so each capture is measured
+# before it is normalised. `sample_inset` shaves the top and bottom margin (where the
+# system bars sit on every profile here) so the measurement covers app content only.
+blank_threshold="${BLANK_THRESHOLD:-0.005}"
+sample_inset=150
+
 raw="$repo/store-assets/raw-screenshots/$form"
 out="$repo/store-assets/screenshots/$form"
 mkdir -p "$out"
@@ -47,8 +56,18 @@ if [ ${#captures[@]} -eq 0 ]; then
 fi
 
 echo "Normalising ${#captures[@]} capture(s) from $raw -> $out"
+blanks=0
 for capture in "${captures[@]}"; do
     name="$(basename "$capture")"
+
+    content_stddev="$(magick "$capture" -shave 0x"$sample_inset" \
+        -colorspace gray -format '%[fx:standard_deviation]' info:)"
+    if awk -v s="$content_stddev" -v t="$blank_threshold" 'BEGIN { exit !(s < t) }'; then
+        printf '  %s  BLANK (content stddev %s) — not normalised\n' "$name" "$content_stddev" >&2
+        blanks=$((blanks + 1))
+        continue
+    fi
+
     magick "$capture" \
         -gravity North -chop 0x"$status_px" \
         -gravity South -chop 0x"$nav_px" \
@@ -60,4 +79,12 @@ for capture in "${captures[@]}"; do
     magick identify -format '%wx%h  %[channels]  %b\n' "$out/$name"
 done
 
+if [ "$blanks" -gt 0 ]; then
+    echo "ERROR: $blanks capture(s) were blank and are missing from $out." >&2
+    echo "       Wake the display and wait for the app's first frame, then re-capture:" >&2
+    echo "         adb shell svc power stayon true" >&2
+    echo "         adb shell input keyevent KEYCODE_WAKEUP" >&2
+    exit 1
+fi
+
 echo "Done."