feat: autotag scoring overhaul, dump-based explore index, and lyrics search

Consolidates in-progress work across autotag, explore, and library:

- autotag: beets/Picard-informed scoring engine — ID-first matching, VA
  handling, recommendation tiers, and a merged distance/rank cascade, with
  an eval harness for regression tracking.
- explore: offline MusicBrainz dump import/incremental refresh replaces the
  legacy tier crawl; index-first local search with fuzzy matching and a
  dedicated ranker; disk-free guards for dump downloads.
- library: artist-credit extraction and matching.
- lyrics: owned-library lyric search (FTS) with LRCLIB backfill.

Also: rewrite README to be user-focused, and migrate upstream to
git.ljones.me/yonlu/yellowjacket.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/library/artistcredit.go

@@ -0,0 +1,72 @@
+package library
+
+import (
+	"strings"
+
+	"yellowjacket/backend/metadata"
+)
+
+// featuringSeparators are the credit join phrases that introduce a
+// featured (non-primary) artist.  Only true "featuring" markers are
+// listed: separators like "&", "x", "with", and "," are deliberately
+// excluded because they routinely appear inside real artist names
+// (e.g. "Simon & Garfunkel", "Tyler, the Creator").
+var featuringSeparators = []string{
+	" feat. ", " feat ", " featuring ", " ft. ", " ft ",
+}
+
+// stripFeaturing returns the credit up to its first "featuring" marker,
+// yielding the primary-artist portion of a credit string.  "Lana Del
+// Rey ft. Sean Lennon" becomes "Lana Del Rey"; a credit with no marker
+// is returned unchanged (trimmed).  Matching is case-insensitive.
+func stripFeaturing(credit string) string {
+	lower := strings.ToLower(credit)
+
+	cut := -1
+
+	for _, sep := range featuringSeparators {
+		if i := strings.Index(lower, sep); i >= 0 && (cut < 0 || i < cut) {
+			cut = i
+		}
+	}
+
+	if cut < 0 {
+		return strings.TrimSpace(credit)
+	}
+
+	return strings.TrimSpace(credit[:cut])
+}
+
+// primaryArtist resolves the single canonical artist a track credit
+// should map to, plus that artist's MusicBrainz ID.  A file tags its
+// ARTIST as a full credit string ("Lana Del Rey ft. Sean Lennon") but
+// carries only one MUSICBRAINZ_ARTISTID — the primary artist's.  Storing
+// the whole credit as an artist entity, and stamping the primary MBID on
+// it, is what produced duplicate, mis-titled artists (one MBID fanned
+// out across many rows); instead we resolve the primary artist's clean
+// name here and keep the full credit only as the artist_credit text.
+//
+// The clean name comes from the album-artist tag when the track resolves
+// to the same MBID as the album artist (the common "Album Artist feat.
+// Guest" case, where ALBUMARTIST is the authoritative name).  Otherwise
+// the featured clause is stripped from the credit string.
+func primaryArtist(tags *metadata.TrackMetadata) (name, mbid string) {
+	mbid = tags.ArtistMBID
+	if mbid == "" {
+		mbid = tags.AlbumArtistMBID
+	}
+
+	if tags.ArtistMBID != "" &&
+		tags.ArtistMBID == tags.AlbumArtistMBID &&
+		tags.AlbumArtist != "" {
+		name = tags.AlbumArtist
+	} else {
+		name = stripFeaturing(tags.Artist)
+	}
+
+	if name == "" {
+		name = "Unknown Artist"
+	}
+
+	return name, mbid
+}