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/library/mbids_test.go

@@ -242,3 +242,22 @@ func TestReadTagMBIDs_Recording(t *testing.T) {
 		t.Errorf("release track only: recording = %q, want empty", got)
 	}
 }
+
+// A Vorbis TRACKNUMBER of "3/20", as Minstrel's importer wrote it, is track 3
+// (#6171): dhowden/tag reads it as 0.
+func TestReadTrackAndDisc(t *testing.T) {
+	for _, tc := range []struct {
+		name        string
+		m           stubMeta
+		track, disc int
+	}{
+		{"vorbis with counts", stubMeta{format: tag.VORBIS, raw: map[string]interface{}{"tracknumber": "3/20", "discnumber": "2/2"}}, 3, 2},
+		{"vorbis bare", stubMeta{format: tag.VORBIS, raw: map[string]interface{}{"tracknumber": " 7 "}}, 7, 0},
+		{"vorbis junk", stubMeta{format: tag.VORBIS, raw: map[string]interface{}{"tracknumber": "A1", "discnumber": "-1"}}, 0, 0},
+		{"id3 is the library's own reading", stubMeta{format: tag.ID3v2_4, raw: map[string]interface{}{"tracknumber": "3/20"}}, 0, 0},
+	} {
+		if track, disc := ReadTrackAndDisc(tc.m); track != tc.track || disc != tc.disc {
+			t.Errorf("%s: got track %d disc %d, want %d and %d", tc.name, track, disc, tc.track, tc.disc)
+		}
+	}
+}