Introduces AppIconMark (navy gradient field + 2×2 glossy blocks reusing
paintGlossyTile) and a testWidgets generator that writes icon.png,
icon_background.png, and icon_foreground.png to assets/icon/ for the
upcoming flutter_launcher_icons task.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>