Soulseek matched-download contexts populate `original_search_result`
with `artist` (singular string) and no `artists` list — the full
multi-artist array lives on `track_info` (the matched Spotify track
object). `extract_source_metadata` only read `original_search.artists`,
so the Soulseek path always fell through to the single-artist branch
and TPE1 ended up with the primary artist only. Deezer-direct
downloads were unaffected because their context populates
`original_search.artists` as a proper list.
Lifted artist resolution into a pure helper
`core/metadata/artist_resolution.py:resolve_track_artists` that walks
`original_search.artists` → `track_info.artists` → `artist_dict.name`
fallback chain. Normalizes mixed list-item shapes (Spotify-style
dicts, bare strings, anything else stringified) and drops empty
entries.
13 new tests pin the resolution order, fallback chain, mixed-shape
normalization, whitespace stripping, and empty/none handling. The
existing `_artists_list` no-fall-through test in
`test_multi_artist_tag_settings.py` was updated to reflect the new
contract (always populated; multi-value write still gated on
`len > 1`) plus a new regression test for the Soulseek shape.
Composes with the existing Deezer per-track upgrade (still fires when
single-artist + track_id available) and feat_in_title /
artist_separator settings (still drive the joined ARTIST string
downstream).
0769fcd5cc