World Countries Tree
A hierarchical tree visualizes the structure of world countries grouped by continent and subregion. The layout uses `d3.tree` and `d3.hierarchy` to position nodes, with `d3.linkHorizontal` generating curved link paths. Text labels display country names with font size decreasing by depth. Pan and zoom functionality is implemented via `d3.zoom`. Data comes from a local `data.json` file, rendered as SVG with a teal stroke and Playfair Display font.