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/explore/fuzzy_test.go

@@ -0,0 +1,43 @@
+package explore
+
+import "testing"
+
+func TestDiceCoefficientTypoTolerance(t *testing.T) {
+	tests := []struct {
+		name    string
+		a, b    string
+		wantMin float64 // score must be at least this
+		wantMax float64 // ...and at most this
+	}{
+		{name: "identical", a: "beatles", b: "beatles", wantMin: 1.0, wantMax: 1.0},
+		{name: "single typo", a: "beetles", b: "beatles", wantMin: 0.5, wantMax: 0.9},
+		{name: "missing char", a: "nirvna", b: "nirvana", wantMin: 0.5, wantMax: 0.95},
+		// Longer-name typo: a single substitution stays comfortably above
+		// the rescue threshold, which is the realistic case (artist and
+		// album names are rarely as short as five characters).
+		{name: "longer name typo", a: "metalica", b: "metallica", wantMin: 0.5, wantMax: 0.95},
+		// Adjacent transposition in a short word is bigrams' known weak
+		// spot — it breaks most windows, so it scores below threshold.
+		// Documented, not a bug: bigram overlap targets substitution,
+		// insertion, and deletion typos.
+		{name: "short transposition", a: "raido", b: "radio", wantMin: 0.0, wantMax: 0.34},
+		{name: "case and space", a: "The Beatles", b: "the beatles", wantMin: 1.0, wantMax: 1.0},
+		{name: "unrelated", a: "beatles", b: "metallica", wantMin: 0.0, wantMax: 0.34},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			got := diceCoefficient(fuzzyBigrams(tc.a), fuzzyBigrams(tc.b))
+			if got < tc.wantMin || got > tc.wantMax {
+				t.Errorf("diceCoefficient(%q, %q) = %.3f, want in [%.2f, %.2f]",
+					tc.a, tc.b, got, tc.wantMin, tc.wantMax)
+			}
+		})
+	}
+}
+
+func TestFuzzyBigramsShortInput(t *testing.T) {
+	if got := fuzzyBigrams("a"); got != nil {
+		t.Errorf("fuzzyBigrams(%q) = %v, want nil", "a", got)
+	}
+}