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_test.go

@@ -0,0 +1,126 @@
+package library
+
+import (
+	"testing"
+
+	"yellowjacket/backend/metadata"
+)
+
+func TestStripFeaturing(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name   string
+		credit string
+		want   string
+	}{
+		{"plain", "Lana Del Rey", "Lana Del Rey"},
+		{"ft dot", "Lana Del Rey ft. Sean Lennon", "Lana Del Rey"},
+		{"feat dot", "2Pac feat. Nate Dogg", "2Pac"},
+		{"featuring", "Beyoncé featuring The Weeknd", "Beyoncé"},
+		{"ft no dot", "Drake ft Travis Scott", "Drake"},
+		{"case insensitive", "Kanye West FEAT. PARTYNEXTDOOR", "Kanye West"},
+		{"ampersand kept", "Simon & Garfunkel", "Simon & Garfunkel"},
+		{"comma kept", "Tyler, the Creator", "Tyler, the Creator"},
+		{"first marker wins", "A feat. B ft. C", "A"},
+		{"trims", "  Daft Punk feat. Panda Bear  ", "Daft Punk"},
+		{"empty", "", ""},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			if got := stripFeaturing(tt.credit); got != tt.want {
+				t.Errorf("stripFeaturing(%q) = %q, want %q", tt.credit, got, tt.want)
+			}
+		})
+	}
+}
+
+func TestPrimaryArtist(t *testing.T) {
+	t.Parallel()
+
+	const lana = "b7539c32-53e7-4908-bda3-81449c367da6"
+
+	tests := []struct {
+		name     string
+		tags     metadata.TrackMetadata
+		wantName string
+		wantMBID string
+	}{
+		{
+			name: "collab on own album uses clean album artist",
+			tags: metadata.TrackMetadata{
+				Artist:          "Lana Del Rey ft. Sean Lennon",
+				AlbumArtist:     "Lana Del Rey",
+				ArtistMBID:      lana,
+				AlbumArtistMBID: lana,
+			},
+			wantName: "Lana Del Rey",
+			wantMBID: lana,
+		},
+		{
+			name: "solo track",
+			tags: metadata.TrackMetadata{
+				Artist:          "Lana Del Rey",
+				AlbumArtist:     "Lana Del Rey",
+				ArtistMBID:      lana,
+				AlbumArtistMBID: lana,
+			},
+			wantName: "Lana Del Rey",
+			wantMBID: lana,
+		},
+		{
+			name: "compilation: album artist differs, strip featuring, keep track mbid",
+			tags: metadata.TrackMetadata{
+				Artist:          "Some Artist feat. Guest",
+				AlbumArtist:     "Various Artists",
+				ArtistMBID:      "aaaa",
+				AlbumArtistMBID: "va-mbid",
+			},
+			wantName: "Some Artist",
+			wantMBID: "aaaa",
+		},
+		{
+			name: "no album artist, strip featuring",
+			tags: metadata.TrackMetadata{
+				Artist:     "Some Artist feat. Guest",
+				ArtistMBID: "aaaa",
+			},
+			wantName: "Some Artist",
+			wantMBID: "aaaa",
+		},
+		{
+			name: "no track mbid falls back to album mbid",
+			tags: metadata.TrackMetadata{
+				Artist:          "Solo",
+				AlbumArtist:     "Solo",
+				AlbumArtistMBID: "album-mbid",
+			},
+			wantName: "Solo",
+			wantMBID: "album-mbid",
+		},
+		{
+			name:     "empty artist",
+			tags:     metadata.TrackMetadata{},
+			wantName: "Unknown Artist",
+			wantMBID: "",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			gotName, gotMBID := primaryArtist(&tt.tags)
+			if gotName != tt.wantName {
+				t.Errorf("primaryArtist name = %q, want %q", gotName, tt.wantName)
+			}
+
+			if gotMBID != tt.wantMBID {
+				t.Errorf("primaryArtist mbid = %q, want %q", gotMBID, tt.wantMBID)
+			}
+		})
+	}
+}