Function stems_folder 

pub fn stems_folder(base: &str) -> String

The .stems sub-folder that sits beside a song’s audio file.

base is the song’s extensionless relative path (the same value the audio and its sidecars are built from), so the folder is {base}.stems. It cannot collide with the audio file ({base}.<ext>) or any {base}.<sidecar> because the .stems suffix is distinct, mirroring the sidecar convention.