Sokhi (again): downloading the base 'Mushoku Tensei S2 Original Soundtrack' embedded
the cour-2 '…サウンドトラック2' cover. numeric_tokens_differ stripped titles to
[a-z0-9], turning CJK into spaces — so the trailing '2' collapsed to a bare '2'
that '第2期' (season 2) already supplied on BOTH sides, leaving the digit sets equal
and the guard blind. Tokenise on \W (Unicode word-aware) instead, so a digit stays
attached to its word ('サウンドトラック2' is its own digit-bearing token). Latin
behaviour is byte-identical (Vol.4 vs Vol.4.5 etc.). Shared guard, so the art picker
AND the MusicBrainz->CAA path are both fixed. Regression tests added.
