feat(metadata): write primary-artist tags in native writers

Native tag writers accept artist_tag_mode=primary and keep only the first credited artist in ARTIST and ALBUMARTIST for every format: FLAC download embedding, FLAC/Opus edits, and the ID3, MP4, APE and RIFF writers. The Android finalizer's FFmpeg fallback applies the same rule.

Rust and Kotlin share one separator set (comma, semicolon, ampersand, a spaced x, feat/ft/featuring/with) and test the same fixtures. The Go-parity split_vorbis pattern is unchanged. Refs #618.

android/app/src/main/kotlin/com/zarz/spotiflac/NativeFinalizationPolicy.kt

@@ -1,6 +1,7 @@
 package com.zarz.spotiflac
 
 import java.util.Locale
+import java.util.regex.Pattern
 import kotlin.math.roundToInt
 
 /**
@@ -373,6 +374,27 @@ internal object NativeFinalizationPolicy {
             ?: ""
     }
 
+    // Same separator set as the Rust writer and Dart's primaryArtistTagValue;
+    // Unicode classes keep \s aligned with their whitespace handling.
+    private val primaryArtistSeparator = Pattern.compile(
+        "\\s*[,;&]\\s*|\\s+x\\s+|\\s+(?:feat(?:uring)?|ft|with)\\.?(?:\\s+|$)",
+        Pattern.CASE_INSENSITIVE or Pattern.UNICODE_CHARACTER_CLASS,
+    ).toRegex()
+
+    /**
+     * Artist tag value for [mode]. Only "primary" changes the value: the first
+     * credited artist for every format. Joined and split values pass through;
+     * the backend writer owns Vorbis splitting.
+     */
+    fun artistTagValue(value: String, mode: String?): String {
+        if (!mode.orEmpty().trim().equals("primary", ignoreCase = true)) return value
+        val trimmed = value.trim()
+        return trimmed.split(primaryArtistSeparator)
+            .map(String::trim)
+            .firstOrNull(String::isNotEmpty)
+            ?: trimmed
+    }
+
     private fun audioFormatForPath(filePath: String, fileName: String): String? {
         for (candidate in listOf(filePath, fileName)) {
             val lower = candidate.trim().lowercase(Locale.ROOT)

android/app/src/main/kotlin/com/zarz/spotiflac/NativeFinalizerMedia.kt

@@ -16,6 +16,7 @@ import com.antonkarpenko.ffmpegkit.ReturnCode
 import com.zarz.spotiflac.SafDownloadHandler.mimeTypeForExt
 import com.zarz.spotiflac.SafDownloadHandler.normalizeExt
 import com.zarz.spotiflac.NativeFinalizationPolicy.applyQualityVariantFilenameLabel
+import com.zarz.spotiflac.NativeFinalizationPolicy.artistTagValue
 import com.zarz.spotiflac.NativeFinalizationPolicy.authoritativeAlbumArtist
 import com.zarz.spotiflac.NativeFinalizationPolicy.displayAudioQuality
 import com.zarz.spotiflac.NativeFinalizationPolicy.formatIndexTag
@@ -314,6 +315,7 @@ internal fun NativeDownloadFinalizer.embedBasicMetadata(context: Context, path:
     val shouldEmbedLyrics = shouldResolveLyrics &&
         NativeFinalizationPolicy.hasUsableLyricsContent(lyrics) &&
         !lyrics.trim().equals("[instrumental:true]", ignoreCase = true)
+    val artistTagMode = input.request.optString("artist_tag_mode", "")
     // FLAC, MP3, Opus, and M4A all have backend tag writers that edit the
     // tag block atomically without an ffmpeg remux (which drops foreign
     // frames and rewrites the whole container). The backend answers
@@ -326,7 +328,7 @@ internal fun NativeDownloadFinalizer.embedBasicMetadata(context: Context, path:
                 .put("artist", artist)
                 .put("album", album)
                 .put("album_artist", albumArtist)
-                .put("artist_tag_mode", input.request.optString("artist_tag_mode", ""))
+                .put("artist_tag_mode", artistTagMode)
                 .put("date", date)
                 .put("isrc", isrc)
                 .put("composer", composer)
@@ -368,11 +370,13 @@ internal fun NativeDownloadFinalizer.embedBasicMetadata(context: Context, path:
     val isOpus = format == "opus"
     val coverFile = if (isM4a || isOpus) downloadCoverForMetadata(context, input) else null
     val labelKey = if (isM4a) "organization" else "label"
+    // The backend writer applies the artist mode itself; this FFmpeg fallback
+    // must store the same primary artist when that mode is selected.
     val metadataPairs = mutableListOf(
         "title" to title,
-        "artist" to artist,
+        "artist" to artistTagValue(artist, artistTagMode),
         "album" to album,
-        "album_artist" to albumArtist,
+        "album_artist" to artistTagValue(albumArtist, artistTagMode),
         "date" to date,
         "track" to trackNumber,
         "disc" to discNumber,