fix: multivalue tags

http/metadata.go

@@ -64,10 +64,8 @@ var canonicalMap = map[string]string{
 var ffmpegKey = map[string]string{
     "Title":       "title",
     "Artist":      "artist",
-    "Artists":     "artist",
     "Album":       "album",
     "AlbumArtist": "album_artist",
-    "AlbumArtists": "album_artist",
     "Composer":    "composer",
     "Track":       "track",
     "TrackNumber": "track",
@@ -80,12 +78,22 @@ var ffmpegKey = map[string]string{
     "Lyrics":      "lyrics",
 }
 
+// ffmpegMultiKey maps multi-valued canonical names to ffmpeg/Vorbis keys.
+// For FLAC/Vorbis, use uppercase ARTISTS/ALBUMARTISTS for proper multi-value support.
+var ffmpegMultiKey = map[string]string{
+    "Artists":      "ARTISTS",
+    "AlbumArtists": "ALBUMARTISTS",
+}
+
 // allowedCanonicals is the set of canonical tags we accept for writes.
 var allowedCanonicals = func() map[string]struct{} {
-    m := make(map[string]struct{}, len(ffmpegKey))
+    m := make(map[string]struct{}, len(ffmpegKey)+len(ffmpegMultiKey))
     for k := range ffmpegKey {
         m[k] = struct{}{}
     }
+    for k := range ffmpegMultiKey {
+        m[k] = struct{}{}
+    }
     return m
 }()
 
@@ -132,6 +140,7 @@ func normalizeAndMapToFFmpeg(in map[string]string) map[string]string {
     return out
 }
 // normalizeMultiToFFmpeg maps multi-valued canonical keys to ffmpeg keys, preserving arrays.
+// Uses ffmpegMultiKey for proper Vorbis comment names (ARTISTS, ALBUMARTISTS).
 func normalizeMultiToFFmpeg(in map[string][]string) map[string][]string {
     out := map[string][]string{}
     for k, arr := range in {
@@ -145,9 +154,14 @@ func normalizeMultiToFFmpeg(in map[string][]string) map[string][]string {
                 continue
             }
         }
-        ffk, ok := ffmpegKey[canonical]
+        // First check if this is a multi-valued tag
+        ffk, ok := ffmpegMultiKey[canonical]
         if !ok {
-            continue
+            // Fall back to regular ffmpeg key
+            ffk, ok = ffmpegKey[canonical]
+            if !ok {
+                continue
+            }
         }
         vals := []string{}
         for _, v := range arr {