feat(19-01): implement WAV RIFF parser/writer and writeWavTags

- Add FormatWAV constant and .wav DetectFormat case
- Add FormatWAV dispatch in pipeline WriteTrackTags switch
- Create wav.go with custom RIFF chunk parser/writer
- parseRIFF: lenient read, RF64 rejection, case-insensitive ID3 chunk detection
- writeRIFF: strict write with correct padding and 4GB size check
- writeWavTags: merge existing ID3v2, reuse applyTextChanges/applyCoverArtChanges
- Atomic write via fileutil.AtomicWrite for crash safety

backend/tagwriter/pipeline.go

@@ -144,6 +144,8 @@ func (tw *TagWriter) WriteTrackTags(trackID int64, changes TagChanges) error {
 		err = writeMp3Tags(tw.logger, audioFile.FilePath, changes)
 	case FormatFLAC:
 		err = writeFlacTags(tw.logger, audioFile.FilePath, changes)
+	case FormatWAV:
+		err = writeWavTags(tw.logger, audioFile.FilePath, changes)
 	default:
 		err = fmt.Errorf("%w: %s", errUnsupportedFormat, format)
 	}