Self-review of the previous commit found a real false-positive risk in
the new filename-bucket pass: two unrelated songs that happen to share
a canonical filename (e.g. ``Yellow.mp3`` by Coldplay vs by some other
artist) would be grouped because all metadata gates were dropped.
The filename pass now layers a safety net under ``require_metadata_match=False``:
- If both rows carry a duration: must agree within 3 seconds. Same
source download = identical duration; a 3+ second gap means
different recordings.
- Else if both rows carry an artist: relaxed 0.6 similarity check —
catches dedup orphans that share an artist tag while rejecting
strangers-with-same-filename.
- Else (no duration AND at least one artist blank): skip — too little
signal to safely group.
5 additional regression tests cover the false-positive prevention
paths plus the genuine dedup-orphan scenarios that must still be
caught after the safety net.
0e68109b68