fix: imported FLAC, Ogg and Opus files keep their track and disc numbers (#6171)

The importer wrote track=N/M for every format, which ffmpeg stores in a
Vorbis comment as TRACKNUMBER=3/20; dhowden/tag reads a Vorbis TRACKNUMBER
with a bare Atoi, so it came back 0. All 245 tracks imported since
2026-10-10 (Humanz, Plastic Beach, the Gorillaz super deluxe and twelve
more albums) have no track or disc number, and their album pages have no
order.

- The importer writes a Vorbis track and disc number alone, with
  TRACKTOTAL and DISCTOTAL, as Picard does. ID3 and MP4 keep N/M.
- ReadTrackAndDisc takes the number before a slash when the Vorbis reader
  returns 0, for the scanner and for the import's own check of a download.
- tagReadVersion 6 re-reads the files already held (tag reads only).

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

internal/importer/verify_test.go

@@ -212,11 +212,21 @@ func TestTagsArgs(t *testing.T) {
 	tg := Tags{Title: "Get Lucky", Artist: "Daft Punk", Track: 8, Tracks: 13, Disc: 1, Discs: 1, Year: 2013,
 		RecordingMBID: "rec-08", GroupMBID: "rg-ram"}
 	flac := strings.Join(tg.args(".flac"), " ")
-	for _, s := range []string{"title=Get Lucky", "track=8/13", "date=2013", "MUSICBRAINZ_TRACKID=rec-08", "MUSICBRAINZ_RELEASEGROUPID=rg-ram"} {
+	for _, s := range []string{
+		"title=Get Lucky", "track=8 ", "TRACKTOTAL=13", "disc=1 ", "DISCTOTAL=1", "date=2013",
+		"MUSICBRAINZ_TRACKID=rec-08", "MUSICBRAINZ_RELEASEGROUPID=rg-ram",
+	} {
 		if !strings.Contains(flac, s) {
 			t.Errorf("flac args lack %q: %s", s, flac)
 		}
 	}
+	// dhowden/tag reads a Vorbis TRACKNUMBER of "8/13" as 0 (#6171).
+	if strings.Contains(flac, "/13") {
+		t.Errorf("flac track number carries its count: %s", flac)
+	}
+	if mp3 := strings.Join(tg.args(".mp3"), " "); !strings.Contains(mp3, "track=8/13") {
+		t.Errorf("mp3 track should be 8/13, which ID3 readers parse: %s", mp3)
+	}
 	if strings.Contains(flac, "MUSICBRAINZ_ALBUMID") {
 		t.Errorf("an empty release id was written: %s", flac)
 	}