On a ballroom stage last month in Providence’s posh Westin Hotel, it happened again. Wearing a flame-red dress, Brown music instructor Judith Lynn Stillman sat at a grand piano and proceeded to inhabit and animate a wildly difficult Débussy piece. White arms flew under the spotlight; scarlet sequins semaphored each flung note. When she finished, just before bursting into applause the audience breathed a collective, stunned wow.