feat: verify scanner engine acceptance criteria

Confirm normalization + 0.85 threshold, IsIgnored skip, per-artist scoping,
and zero-threshold→default contract. Added TestScanArtist_ZeroThresholdUsesDefault,
fixed gofmt on two test files. Coverage: scanner 90.3%, normalize 100.0%.

internal/scanner/scan_test.go

@@ -82,6 +82,30 @@ func TestScanArtist(t *testing.T) {
 	}
 }
 
+func TestScanArtist_ZeroThresholdUsesDefault(t *testing.T) {
+	db := newTestDB(t)
+	defer db.Close()
+
+	seedArtist(t, db, "artist-1", "Pink Floyd")
+	seedLocalAlbum(t, db, "l1", "artist-1", "The Wall")
+	seedExternalRelease(t, db, "rg1", "artist-1", "The Wall", false)
+	seedExternalRelease(t, db, "rg2", "artist-1", "Animals", false)
+
+	// Pass 0 (zero value / unset) and the explicit default; results must match.
+	zero, err := ScanArtist(context.Background(), db, "artist-1", 0)
+	if err != nil {
+		t.Fatalf("ScanArtist(0) error: %v", err)
+	}
+	explicit, err := ScanArtist(context.Background(), db, "artist-1", DefaultThreshold)
+	if err != nil {
+		t.Fatalf("ScanArtist(%v) error: %v", DefaultThreshold, err)
+	}
+	if len(zero) != len(explicit) {
+		t.Errorf("ScanArtist(0) returned %d missing, ScanArtist(%v) returned %d; must match",
+			len(zero), DefaultThreshold, len(explicit))
+	}
+}
+
 func TestScanArtist_IgnoredNotReported(t *testing.T) {
 	db := newTestDB(t)
 	defer db.Close()