A 72-year-old woman with a 15-year history of multiple myeloma (IgG lambda) is found to have progressive dyspnea and orthopnea. Echocardiography shows restrictive physiology with preserved ejection fraction. Serum free light chain ratio is markedly elevated (lambda 850 mg/L, kappa 15 mg/L). Which investigation is most specific for confirming cardiac amyloidosis?