certify 0.1.0

• Initial release.
• make_certificate() renders a landscape PDF certificate with decorative borders, corner ornaments, optional laurels, an optional logo, and one or two signature blocks.
• cert_theme() constructs a color palette; convenience presets cert_theme_classic(), cert_theme_formal(), and cert_theme_warm() cover common looks.
• device = c("cairo_pdf", "pdf") argument lets the caller fall back to the base pdf device on systems without Cairo (e.g. macOS without XQuartz, stripped Linux containers).