Write a design pattern specification for a Split-Button Dropdown component in a {{tone}} tone for {{target_audience}}. The implementation framework is {{framework}}. The goal is {{goal}}. Output must follow corporate/enterprise design system principles. Cover these sections: Pattern Overview, When to Use, When NOT to Use, Anatomy & Layout, Behavior Specification, Content Guidelines, Accessibility Specification, Design Variations, Code Integration Notes, Real-World Examples, and Quality Checklist. Keep under 1800 words. Use Markdown with headers and bullet points. Bold key terms. Skip any section if information is unknown. Paste any relevant background information after these instructions before running.