:sig
String path_join(String base, String child)

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

:desc
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. This makes `path_join()` a better fit for app-level path assembly than open-coded string concatenation.

:see
>sys

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