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_classify.go

@@ -1,7 +1,9 @@
 package library
 
 import (
+	"path/filepath"
 	"regexp"
+	"sort"
 	"strings"
 	"unicode"
 )
@@ -90,6 +92,35 @@ func ClassifyDuplicateGroup(tier string, members []ClassifyMember) DuplicateClas
 var versionSuffix = regexp.MustCompile(
 	`(?i)\s-\s[^-]*\b(version|instrumental|remix|mix|live|acoustic|demo|edit|reprise|rework|vip)\b[^-]*$`)
 
+// versionWord matches a word that names a version of a song.
+var versionWord = regexp.MustCompile(
+	`(?i)\b(clean|explicit|instrumentals?|live|acoustic|unplugged|demo|remix|mix|edit|radio|karaoke|a ?cappella|acapella|reprise)\b`)
+
+// versionWords lists, sorted and lowercased, the version words in a copy's
+// title and file name together. Two copies holding one title tag can still be
+// two versions, told apart only by the file name: "Suite-Pee" and
+// "Suite-Pee (Clean Version)", "Luv (sic) Grand Finale" and its instrumental.
+func versionWords(title, path string) string {
+	seen := map[string]bool{}
+	name := strings.TrimSuffix(filepath.Base(path), filepath.Ext(path))
+	for _, w := range versionWord.FindAllString(title+" "+name, -1) {
+		w = strings.ToLower(w)
+		switch {
+		case w == "instrumentals":
+			w = "instrumental"
+		case strings.Contains(w, "cappella") || w == "acapella":
+			w = "acappella"
+		}
+		seen[w] = true
+	}
+	words := make([]string, 0, len(seen))
+	for w := range seen {
+		words = append(words, w)
+	}
+	sort.Strings(words)
+	return strings.Join(words, ",")
+}
+
 // songKey is MatchTitleKey with every bracketed segment and a dashed version
 // suffix dropped: the song a title names, whatever version of it.
 // "Lovesick (Instrumental)", "Lovesick - Instrumental Version" and "Lovesick"