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.

rust_backend/crates/core/tests/artist_tag_mode.rs

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+use spotiflac_core::tags::{embed_flac_metadata, read_audio_tags, rewrite_audio_tags};
+use std::collections::BTreeMap;
+use std::io::Cursor;
+
+const PAYLOAD: &[u8] = b"\xff\xf8\x12\x34unchanged audio payload";
+const CREDITS: &str = "Artist A, Artist B & Artist C";
+
+fn atom(kind: &[u8; 4], body: &[u8]) -> Vec<u8> {
+    [
+        ((body.len() + 8) as u32).to_be_bytes().as_slice(),
+        kind,
+        body,
+    ]
+    .concat()
+}
+
+fn ogg_page(packet: &[u8], sequence: u32, flags: u8) -> Vec<u8> {
+    let mut header = vec![0; 27];
+    header[..4].copy_from_slice(b"OggS");
+    header[5] = flags;
+    header[14..18].copy_from_slice(&1_u32.to_le_bytes());
+    header[18..22].copy_from_slice(&sequence.to_le_bytes());
+    header[26] = 1;
+    header.push(packet.len() as u8);
+    header.extend(packet);
+    header
+}
+
+fn source(format: &str) -> Vec<u8> {
+    match format {
+        "flac" => [b"fLaC\x80\0\0\x22".as_slice(), &[0; 34], PAYLOAD].concat(),
+        "mp3" | "ape" => PAYLOAD.to_vec(),
+        "m4a" => [
+            atom(b"ftyp", b"M4A \0\0\0\0"),
+            atom(b"moov", &[]),
+            atom(b"mdat", PAYLOAD),
+        ]
+        .concat(),
+        "opus" => [
+            ogg_page(b"OpusHead\x01\x02\0\0\x80\xbb\0\0\0\0\0", 0, 2),
+            ogg_page(b"OpusTags\0\0\0\0\0\0\0\0", 1, 0),
+            ogg_page(PAYLOAD, 2, 4),
+        ]
+        .concat(),
+        "wav" | "aiff" => {
+            let aiff = format == "aiff";
+            let mut body = if aiff { b"AIFFSSND" } else { b"WAVEdata" }.to_vec();
+            let length = PAYLOAD.len() as u32;
+            body.extend(if aiff {
+                length.to_be_bytes()
+            } else {
+                length.to_le_bytes()
+            });
+            body.extend(PAYLOAD);
+            if length % 2 == 1 {
+                body.push(0);
+            }
+            let length = body.len() as u32;
+            [
+                if aiff { b"FORM" } else { b"RIFF" }.as_slice(),
+                &if aiff {
+                    length.to_be_bytes()
+                } else {
+                    length.to_le_bytes()
+                },
+                &body,
+            ]
+            .concat()
+        }
+        _ => unreachable!(),
+    }
+}
+
+fn tagged(format: &str, mode: &str) -> (String, String) {
+    let fields = BTreeMap::from([
+        ("title".into(), "Example".into()),
+        ("artist".into(), CREDITS.into()),
+        ("album_artist".into(), CREDITS.into()),
+        ("artist_tag_mode".into(), mode.into()),
+    ]);
+    let mut output = Vec::new();
+    rewrite_audio_tags(
+        &mut Cursor::new(source(format)),
+        &mut output,
+        format,
+        &fields,
+        None,
+        &|| Ok(()),
+    )
+    .unwrap();
+    let tags = read_audio_tags(&mut Cursor::new(&output), format, &|| Ok(())).unwrap();
+    (tags.artist, tags.album_artist)
+}
+
+#[test]
+fn primary_mode_keeps_only_the_first_artist_in_every_native_writer() {
+    for format in ["flac", "mp3", "m4a", "opus", "ape", "wav", "aiff"] {
+        assert_eq!(
+            tagged(format, "primary"),
+            ("Artist A".into(), "Artist A".into()),
+            "{format}"
+        );
+        // Mode values are matched like split_vorbis: trimmed, any case.
+        assert_eq!(tagged(format, " Primary ").0, "Artist A", "{format}");
+        assert_eq!(
+            tagged(format, "joined"),
+            (CREDITS.into(), CREDITS.into()),
+            "{format}"
+        );
+    }
+}
+
+#[test]
+fn primary_mode_applies_to_flac_download_embedding() {
+    let fields = BTreeMap::from([
+        ("TITLE".into(), "Example".into()),
+        ("ARTIST".into(), "Calle 24, Chino Pacas".into()),
+        ("ALBUMARTIST".into(), "Calle 24 feat. Chino Pacas".into()),
+    ]);
+    let mut output = Vec::new();
+    embed_flac_metadata(
+        &mut Cursor::new(source("flac")),
+        &mut output,
+        &fields,
+        "primary",
+        None,
+        &|| Ok(()),
+    )
+    .unwrap();
+    let tags = read_audio_tags(&mut Cursor::new(&output), "flac", &|| Ok(())).unwrap();
+    assert_eq!(tags.artist, "Calle 24");
+    assert_eq!(tags.album_artist, "Calle 24");
+    assert!(output.ends_with(PAYLOAD));
+    let comments = String::from_utf8_lossy(&output);
+    assert_eq!(
+        comments.matches("ARTIST=").count(),
+        2,
+        "one ARTIST, one ALBUMARTIST"
+    );
+}