Decide FileTreeNode public API surface for consumers

[veganstraightedge]

• What properties are exposed to consumers? The closures for `onFileSelect`, `onFileOpen`, `.rowContent`, `.contextMenu` all receive some representation of a node. Is it just a `URL`, or a richer type with properties like `isFolder`, `name`, `sourceControlStatus`, `isOpen`, `isModified`, `depth`?
• Should consumers get a `FileTreeNode` or a simpler `FileTreeItem` protocol/struct? Exposing the full `@Observable` node risks consumers mutating internal state. A read-only snapshot or protocol might be safer.