fix: a duplicate fold never deletes a lossless copy for a lossy one, nor folds two versions (M498)

The first album-copy fold pass on 2026-10-20 removed five FLACs (Måneskin
Rush!, Weezer 2016) in favour of mp3s whose only edge was a recording MBID,
and an explicit "Suite-Pee" for its clean version. ProposeSurvivor now ranks
lossless before tag fit; tag fit still outranks file size, which is what the
Humanz rips needed. The fold keys copies by their version words (title and
file name), so a clean, live or instrumental copy holding the same title tag
is linked, not folded.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/library/duplicate_survivor.go

@@ -52,16 +52,18 @@ var losslessFormats = map[string]bool{"flac": true, "wav": true}
 // the report shows it and the merge (#3911) lets the operator choose another.
 //
 // In order:
-//  0. the copy whose tags fit the album best (TagFitScore). It is zero for
+//  1. lossless over lossy — the one difference no later step can recover. It
+//     comes before tag fit: tags can be fixed, a deleted FLAC cannot, and on
+//     2026-10-20 five FLACs went for mp3s that only carried a recording MBID
+//  2. the copy whose tags fit the album best (TagFitScore). It is zero for
 //     every copy when the caller does not know it, and the order falls through
-//     to the quality rules below
-//  1. lossless over lossy — the one difference no later step can recover
-//  2. the larger file — for one recording at one duration that is the higher
+//     to the rules below
+//  3. the larger file — for one recording at one duration that is the higher
 //     bitrate. The scanner does not record bitrate (tracks.bitrate is never
 //     filled), so file size is the signal that actually exists
-//  3. the copy in the library longest — the one most likely to carry the play
+//  4. the copy in the library longest — the one most likely to carry the play
 //     history and likes, so the merge moves the least
-//  4. the lowest track id, so the choice is stable between page loads
+//  5. the lowest track id, so the choice is stable between page loads
 func ProposeSurvivor(cands []SurvivorCandidate) (trackID, reason string) {
 	if len(cands) == 0 {
 		return "", ""
@@ -77,10 +79,10 @@ func ProposeSurvivor(cands []SurvivorCandidate) (trackID, reason string) {
 	// runner-up — the rule that actually decided, not every rule it passed.
 	next := ranked[1]
 	switch {
-	case best.TagFit != next.TagFit:
-		return best.TrackID, "tags fit the album"
 	case isLossless(best) != isLossless(next):
 		return best.TrackID, "lossless (" + strings.ToLower(best.FileFormat) + ")"
+	case best.TagFit != next.TagFit:
+		return best.TrackID, "tags fit the album"
 	case best.FileSize != next.FileSize:
 		return best.TrackID, "largest file"
 	case !best.AddedAt.Equal(next.AddedAt):
@@ -91,12 +93,12 @@ func ProposeSurvivor(cands []SurvivorCandidate) (trackID, reason string) {
 }
 
 func survivorBefore(a, b SurvivorCandidate) bool {
-	if a.TagFit != b.TagFit {
-		return a.TagFit > b.TagFit
-	}
 	if isLossless(a) != isLossless(b) {
 		return isLossless(a)
 	}
+	if a.TagFit != b.TagFit {
+		return a.TagFit > b.TagFit
+	}
 	if a.FileSize != b.FileSize {
 		return a.FileSize > b.FileSize
 	}