“A real-life drama of succession as riveting as any Richard Wagner opera is casting a shadow over this year’s Bayreuth Festival, raising the anticipation level among devotees of the German master even before the first curtain rises Wednesday at the musical shrine he inaugurated 131 years ago. No matter what the outcome, chances are good that whoever heads the famous German festival — a traditional battleground for Wagner clan members vying for influence — will be part of the family for some time to come. But which Wagner will it be?”