smtp_send: conecta+envia+cierra en un paso via smtplib (TLS/STARTTLS/plain).
email_build_html: construye EmailMessagePy frozen dataclass con cuerpo HTML.
Solo stdlib Python: smtplib, email.mime. Tests con mock SMTP server threading.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>