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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>