:sig
String path_join(String base, String child)

:params
base : parent path
child : child path or absolute override
return value : combined path

:see
>sys

:content
Joins two filesystem-style path fragments with a single `/` when needed.

If `child` is empty, `base` is returned. If `child` already starts with `/`, it is returned unchanged. That makes `path_join()` a better fit for app-level path assembly than open-coded string concatenation.

Related:

- PHP: manual path composition with `DIRECTORY_SEPARATOR` or helper wrappers around it
- JavaScript / Node.js: Node `path.join()`