test(library): skip the cover-tier scan test when fixtures are absent

TestScan_StoresOnlyCoverTiers built the fixture path by hand, so in a
tree where make testdata had not run it failed on a missing covers
directory, where every other fixture test skips via testfixtures.Load.
In a fresh worktree that failure blocked the pre-push hook for every
branch.

Closes #266

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017HJiuc3ZZhxsPXz3ozTirT

backend/library/coverart_storage_test.go

@@ -8,6 +8,7 @@ import (
 
 	"yellowjacket/backend/coverart"
 	"yellowjacket/backend/database/sql/sqlcgen"
+	"yellowjacket/internal/testfixtures"
 )
 
 // TestScan_StoresOnlyCoverTiers pins the size decision: a scan writes
@@ -26,10 +27,9 @@ func TestScan_StoresOnlyCoverTiers(t *testing.T) {
 
 	lib, db := setupTestLibrary(t)
 
-	root, err := filepath.Abs("../../test_data/music_library_test")
+	// Load skips when the fixture library has not been generated, as
-	if err != nil {
+	// every other fixture test does.
-		t.Fatalf("resolve fixture path: %v", err)
+	root := testfixtures.Load(t).Root()
-	}
 
 	library, err := db.Queries.CreateLibrary(lib.ctx, sqlcgen.CreateLibraryParams{
 		Name: "Fixtures",