test(coverart): no-MBID enrich settles 'none' (per canonical behavior)

Operator decision: the enricher is canonical. No MBID still runs the
provider chain (name-based providers — Deezer/Last.fm — resolve
without an MBID); if every provider returns ErrNotFound the row
settles cover/artist source 'none' at the current sources version
(re-eligible only when the registered provider set changes). It does
NOT skip-and-leave-NULL.

The two _NoMBID_LeavesNull tests predated the name-based providers
(0020 slice) and asserted the old skip→NULL contract. Updated:
- TestEnrichArtist_NoMBID_SettlesNone: stub now returns ErrNotFound
  (realistic MBID-only-provider-with-empty-MBID), expect source 'none'.
- TestEnrichAlbum_NoSidecarNoMBID_SettlesNone: empty registry →
  allWere404 stays true → expect 'none'.

Last failing cluster from the CI-integration initiative; suite should
now be fully green.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/coverart/enricher_test.go

@@ -121,7 +121,7 @@ func TestEnrichAlbum_SidecarFound(t *testing.T) {
 	}
 }
 
-func TestEnrichAlbum_NoSidecarNoMBID_LeavesNull(t *testing.T) {
+func TestEnrichAlbum_NoSidecarNoMBID_SettlesNone(t *testing.T) {
 	pool := newPool(t)
 	id, _ := seedAlbumWithTrack(t, pool, "NoMbid", "Artist", "")
 	resetRegistryForTests() // deterministic empty registry (no provider can supply)
@@ -134,8 +134,11 @@ func TestEnrichAlbum_NoSidecarNoMBID_LeavesNull(t *testing.T) {
 	if err != nil {
 		t.Fatalf("get: %v", err)
 	}
-	if row.CoverArtSource != nil {
-		t.Errorf("source = %v, want nil (NULL — eligible for retry on next scan)", row.CoverArtSource)
+	// No sidecar, no MBID, no provider yields → the chain settles
+	// 'none' (allWere404 stays true), version-stamped so it is only
+	// re-tried when the registered provider set changes.
+	if row.CoverArtSource == nil || *row.CoverArtSource != "none" {
+		t.Errorf("source = %v, want 'none' (settled)", row.CoverArtSource)
 	}
 }