Render a nested dict as a tree-style listing. Directories are dicts (name → children), files map to None. Implement render_tree(name, node) returning a string:
project
├── README.md
└── src
├── app.py
└── utils
└── io.py
Within a directory, list subdirectories first, then files, each group sorted alphabetically. Use ├── / └── connectors and │ / for indentation.