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/musicbrainz/sync_test.go

@@ -888,4 +888,3 @@ func TestSyncArtistDiscography_ResyncWithNotifications(t *testing.T) {
 		t.Errorf("expected 0 notifications after resync, got %d", count)
 	}
 }
-