When users bind the same host music directory into both SoulSync
(e.g. /app/Transfer) and a media server like Plex (e.g.
/media/Music), both scans add a track row pointing at the same
physical file via different mount paths. The detector previously
flagged those as duplicate groups even though there's only one
file on disk.
New _is_same_physical_file helper filters pairs where:
- The trailing 3 path segments match (filename + album + artist
folder), so they're the same release on disk.
- The leading mount roots actually differ.
- Durations agree within 1s when both rows carry duration data.
Adds 10 regression tests covering the reported scenario plus
edge cases (Windows separators, case differences, missing
durations, sibling-album false-positive guard).
