Fix case-sensitive comparison in compare_strings fuzzy matching (#3151)

In non-strict mode, compare_strings lowercases str1 but passes the
original-case str2 to SequenceMatcher on line 564. This causes fuzzy
matching to penalize case differences, making it fail to match strings
like "Track Feat. John" vs "TRACK FT. JOHN" that should be considered
equivalent.

Also fix the elif branch (line 559) to replace on str2_lower instead
of str2, so the result is consistently lowered.

Co-authored-by: David Bishop <[email protected]>
Co-authored-by: Claude Opus 4.6 <[email protected]>