is_disc_folder() was matching any folder name containing "Disc N" or
"CDN" anywhere in the name. This caused albums like "100 Popular
Classics, Disc 1" or "Now 75 - CD1" under Various Artists to be treated
as disc subfolders, collapsing all tracks into a single album group.
Now is_disc_folder() only returns true when the folder name consists
exclusively of disc-related tokens (disc/cd/disk, digits, and modifiers
like "bonus"). Album names with extra words are correctly left as-is.
Users must re-scan their library for the fix to take effect.