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_test.go

@@ -90,10 +90,11 @@ func TestProposeSurvivor_ReasonIsTheDecidingRule(t *testing.T) {
 	}
 }
 
-// M498 #5436: the copy whose tags fit the album outranks quality, which decides
-// only between copies whose tags fit equally. Each case is built so the quality
-// rules would choose the other copy, so a reorder that drops tag fit fails.
-func TestProposeSurvivor_TagFitFirst(t *testing.T) {
+// M498 #5436: the copy whose tags fit the album outranks every quality rule but
+// lossless, which decides first: a deleted FLAC cannot be recovered, and on
+// 2026-10-20 five went for mp3s whose only edge was a recording MBID. Each tag
+// fit case is built so file size would choose the other copy.
+func TestProposeSurvivor_LosslessThenTagFit(t *testing.T) {
 	at := time.Date(2025, 1, 1, 0, 0, 0, 0, time.UTC)
 	cases := []struct {
 		name       string
@@ -102,10 +103,18 @@ func TestProposeSurvivor_TagFitFirst(t *testing.T) {
 		wantReason string
 	}{
 		{
-			name: "tag fit beats lossless",
+			name: "lossless beats tag fit",
 			cands: []SurvivorCandidate{
-				{TrackID: "flac", FileFormat: "flac", FileSize: 40_000_000, AddedAt: at, TagFit: 1},
 				{TrackID: "fits", FileFormat: "mp3", FileSize: 5_000_000, AddedAt: at, TagFit: 3},
+				{TrackID: "flac", FileFormat: "flac", FileSize: 40_000_000, AddedAt: at, TagFit: 1},
+			},
+			wantID: "flac", wantReason: "lossless (flac)",
+		},
+		{
+			name: "tag fit beats a larger file",
+			cands: []SurvivorCandidate{
+				{TrackID: "big", FileFormat: "mp3", FileSize: 9_000_000, AddedAt: at, TagFit: 1},
+				{TrackID: "fits", FileFormat: "m4a", FileSize: 5_000_000, AddedAt: at, TagFit: 3},
 			},
 			wantID: "fits", wantReason: "tags fit the album",
 		},