feat(20-01): implement OGG Vorbis tag writer with custom page parser and CRC32

- Custom OGG page parser/writer with MSB-first CRC32 lookup table (ogg.go)
- Vorbis Comment packet parse/serialize with raw byte preservation (ogg_vorbis.go)
- METADATA_BLOCK_PICTURE base64 encoding for cover art with legacy field stripping
- Multi-stream and non-Vorbis OGG rejection with clear error messages
- Pipeline integration: FormatOGG constant, .ogg in DetectFormat, writeOggTags dispatch
- Lenient-read/strict-write: warn on CRC mismatch, always write correct CRCs
- Page sequence renumbering and crash-safe writes via AtomicWrite

backend/tagwriter/tagwriter.go

@@ -38,6 +38,8 @@ const (
 	FormatFLAC AudioFormat = "flac"
 	// FormatWAV is the WAV audio format.
 	FormatWAV AudioFormat = "wav"
+	// FormatOGG is the OGG Vorbis audio format.
+	FormatOGG AudioFormat = "ogg"
 )
 
 // errUnsupportedFormat is returned when the audio format is not supported.
@@ -54,6 +56,8 @@ func DetectFormat(filePath string) (AudioFormat, error) {
 		return FormatFLAC, nil
 	case ".wav":
 		return FormatWAV, nil
+	case ".ogg":
+		return FormatOGG, nil
 	default:
 		return "", fmt.Errorf("%w: %s", errUnsupportedFormat, ext)
 	}