cat2151/clap-mml-render-tui

AI Summary: The `src/main.rs` file has grown beyond 500 lines and needs refactoring to improve maintainability and readability. The plan is to split its responsibilities into dedicated modules: `src/cli.rs` for argument parsing, `src/mml_render.rs` for MML rendering logic, `src/tui.rs` for TUI drawing, and a streamlined `src/main.rs` for overall application integration. This adheres to the Single Responsibility Principle, localizing changes and clarifying code structure.